I agree with BTDT on the mobile Navionics being nicer than the one that I have on my HB 859. Both get me to where I want to go on familiar waters.
The mobile is interactive, or at least was/is, if you do the update. I haven't, since I wasn't sure if updating would charge me again, or take away my old app if I didn't pay... I use as a back up or if I am off of the water and curious about a spot.
BT - Did you have to pay for the update, or was it part of the original price?
I think that I paid $10 about 6-7 years ago, before I had a fish finder with GPS (and my hand held GPS decided to stop working). Back when I would refuse to pay anything for an app (which I mostly still do). But in comparison to the cards, it is a bargain. That said, the chip that was in my handheld ended up working in my HB 859... So, that saved me a few $, but not in HD, which would be nice, if it is even offered.