I fished for many years using mono, then tried some braid and saw the benefits.
After a long time off, (not fishing due to life responsibilities) I see that now there is a trend for using braid with a fluorocarbon leader to achieve (apparently) the sensitivity of braid feel (no stretch) with fluoro "invisibility".
We can argue all of the points on each but I'm asking as an avid sportsman prefering to use the best available when reasonable but having not kept up on the transition to this system.
I fish 100% fresh water, all species from bluegills to pike/musky, use both baitcasters and spinning gear. I prefer to cast most of the time but I do troll when appropriate. I'm not a catfisher and while I would like to fish for salmon, I'm in the middle of the country where that opportunity doesn't present itself.
The best simplified explanation of this setup has been here 
Since I've recently tried this setup, having tried to educate myself of the benefits I have the following questions.
1- Is it worth the effort to use this type of setup? It seems overly complex to have to tie a "brittle' fluoro to braid knot leader that has to travel through rod eyelets weakening it with each cast
2- Since most of the freshwater fish rarely take more than a few runs most spools are filled with backer or primary line that never see's the light of day past 50 or 75 yards. Many 2500 to 3500 series spinning reels carry from 200 to 400 yards of line likewise baitcasters carry at least an extra 50 or more yards of "spare room". Would it make sense to just use fluoro on the last 50 yards? If fluoro is as superior as suggested to mono, and if (as I suggest) having to cast a knot through eyelets weakens the knots ability, then why not eliminate the damaging portion by placing the knot back where it almost never sees the eyelets.
Has anyone done a study on the knot strength of Fluoro to Braid after 100's casts through eyelets and dragged through sand and rocks? I know regular checking is important but retie-ing 20 pound slippery braid to 8 pound fluoro isn't as easy as it appears on YOUTUBE when they show you with 100 pound HiVis.
 
Any contributions, comments, suggestions, experiences appreciated.
 

Like rod/reel selection, “the best setup” is specific for the fishing you are doing. Braid with a leader works for me on many types of fishing, but might not be “the best” for many situations and probably not advisable for some. If you are vertical jigging from a boat, want the benefits of sensitivity of braid and low visibility of floro, the knot isn’t a big deal as it would be casting. For that situation you can go with a longer leader too. If wanting to cast far and drift lighter baits, shorten the leader so that it isn’t going through the eyelets. For this, maybe straight mono or straight floro is better. It really depends on how light the bait and how far the casts needed. If the bite seems to be affected by fish seeing your braid, might need to go longer/lighter leader. Lighter the leader, smaller the knot and not as much issue with going through eyelets. This flexibility of changing out leader diameter and length is one thing that I like about braid and leaders that I really didn’t consider at first. I like high vis braid too. It is excellent when doing finesse fishing, often detecting lighter takes seeing the line moving like a big long bobber. As light of braid as you can find for panfish/trout rod. Size up braid size slightly for bass rod. Size up slightly for bigger (salt water). Unless shark fishing, the 60# or other heavy stuff isn’t necessary for most. I’m sure guys that do other specific types of fishing have good use for heavy line, but I think for the average angler, the lighter the better. Let your drag do its job.

Uni to uni for braid to floro leader is simple to tie and rarely fails. Practice knots with old line until you figure it out. Plenty of knot strength reviews online.

I use 12-24" of flouro at the end of my braid for most freshwater applications. Uni splice, micro swivel, or Albright knot depending on application/line diameters. I don't cast the splice through the guides typically.

Uni to uni knot is easy to tie.  I use it to connect my 50/60# mono top shot to my 65# braid on my shark rods.  Wouldn't be much different with fluoro.  That knot goes through the eyes, over the sand bars and handles smoking runs of the sharks.  NEVER had that knot fail.  Had other terminal tackle and knot failures at the hooks but never the splice knot.  I stick with the mono top shop on those though to give a little more stretch with when an aggressive shark really hits hard.  Helps absorb the shock.  For lighter applications, I would have no problem with using the braid to fluoro uni to uni knot.  Used to tie the improved albright years back but the uni seems to work just fine and quicker to tie.  

I have had it fail, but very likely from user error, not wetting the line enough, bad leader material or just too light of leader. I switched from a surgeons knot that was failing way too often. Good knot for mono to mono or floro, doesn’t seem to work well with braid for me at least. The amount of times that I have had uni to uni fail in comparison isn’t close. I can’t recall having it fail on heavier setups. I think many are when I size down leader when bite is slow and forget to adjust my drag to make up for it.

For braid to flouro or mono, I use the uni to uni most of the time as it's easy to tie quickly. Now I'm gonna be doin' the FG knot, also relatively easy to tie except in a boat on choppy water with the wind blowin' 15 mph. In both knot cases, remember to double the number of wraps of the braid versus the flouro or mono. Braid is extremely slick. Mono leader if you want to stay up more in the water column; flouro if you're bottom fishin'.

I use a 3x or 4x surgeons knot for braid to flouro. I feel like this is not a very popular knot to use for this purpose and I can not for the life of me figure out why.

It is SO much easier to tie than any of those other popular knots, like the double uni, or FG, or a blood knot. And it’s pretty damm strong. When I have to break off, 90% of the time the knot at the bait fails before the knot at the leader, and I use a palomar for the bait which is also a pretty reliable knot.

Maybe it’s the profile, and it doesn’t go through the guides well enough for some people?

I use Fireline with a fluro leader and when I 1st started using this setup I quickly discovered that Fireline is abrasive and when it is tied to fluro, it will cut thru the soft fluro like a hot knife thru butter as soon as pressure is applied. Bye bye fish and lure. I soon learned to use a tiny swivel to connect instead of knots and this works fine, you just have to remember not to reel it thru the guides. Learn to make all casts with the entire length of leader dangling from the rod tip. 

Conococheague
I use Fireline with a fluro leader and when I 1st started using this setup I quickly discovered that Fireline is abrasive and when it is tied to fluro, it will cut thru the soft fluro like a hot knife thru butter as soon as pressure is applied. Bye bye fish and lure. I soon learned to use a tiny swivel to connect instead of knots and this works fine, you just have to remember not to reel it thru the guides. Learn to make all casts with the entire length of leader dangling from the rod tip. 

What sizes are you using? I think the slice through is more of an issue on light lines, at least for me. I had horrible issues with 4# leader on I think 10# braid, might be thinner. I’m talking panfish breaking…. This was using surgeons knots. My guess is they weren’t tight enough or friction burn from not wetting or pulling knots too fast. I’ve pretty much changed my leader material to 8# or higher now. I haven’t trout fished since, but if I do, definitely doing the swivel thing. I did this for panfishing a little until I just went to 8# leader and fish didn’t seem to mind. Helps me get out of some thicker weeds and timber too.

Not as much of an issue on heavier lines but I had and was enough. After losing 3-4 wipers fishing with Pensfan a few years ago, had me swearing off the surgeons knot. Since changing to uni to uni, which is just as easy to tie as the surgeon after practice, I rarely have knot breaks. The ones that I do, I am betting had to do with either not pulling them tight enough or more likely not wetting enough and pulling too fast. I just get too antsy tying when the fish are biting. Nothing beats the feeling, but turns me into an impatient child rush the tying. I’d bet 90% of my knot fails were tied a few minutes after getting a bite. The ones that I tie at home seem to work great. Go figure.

Porktown

Conococheague
I use Fireline with a fluro leader and when I 1st started using this setup I quickly discovered that Fireline is abrasive and when it is tied to fluro, it will cut thru the soft fluro like a hot knife thru butter as soon as pressure is applied. Bye bye fish and lure. I soon learned to use a tiny swivel to connect instead of knots and this works fine, you just have to remember not to reel it thru the guides. Learn to make all casts with the entire length of leader dangling from the rod tip. 

What sizes are you using? I think the slice through is more of an issue on light lines, at least for me. I had horrible issues with 4# leader on I think 10# braid, might be thinner. I’m talking panfish breaking…. This was using surgeons knots. My guess is they weren’t tight enough or friction burn from not wetting or pulling knots too fast. I’ve pretty much changed my leader material to 8# or higher now. I haven’t trout fished since, but if I do, definitely doing the swivel thing. I did this for panfishing a little until I just went to 8# leader and fish didn’t seem to mind. Helps me get out of some thicker weeds and timber too.

Not as much of an issue on heavier lines but I had and was enough. After losing 3-4 wipers fishing with Pensfan a few years ago, had me swearing off the surgeons knot. Since changing to uni to uni, which is just as easy to tie as the surgeon after practice, I rarely have knot breaks. The ones that I do, I am betting had to do with either not pulling them tight enough or more likely not wetting enough and pulling too fast. I just get too antsy tying when the fish are biting. Nothing beats the feeling, but turns me into an impatient child rush the tying. I’d bet 90% of my knot fails were tied a few minutes after getting a bite. The ones that I tie at home seem to work great. Go figure.

I was using 6lb Fireline to 6lb fluro leader, tied with a uni to uni knot. Broke off by a trout about 10" long. I was also dumb enough to try using 8lb Fireline to 8lb fluro tied by uni to uni for Steelhead. I lost most of the lures I had with me that trip. i don't have this issue with Power Pro or other types of braid I tried, but Fireline is not a true braid and it is abrasive. I've had it slice my fingers up many times when trying to free a snagged hook.