Navionics here.
Same here.
I've tried a few others, and I keep coming back to Navionics:
1) The user interface is more intuitive (to me) than anything else I've tried.
2) Automatic dock-to-dock routing - of course the routes aren't perfect, and you always want to review themn. But the ability to quickly estimate 'going from A to B will take about 3.5 hours, and A to C would be 4.5' is really nice. We're all addicted to mapping systems for driving, and this is the closest I've seen in a boating app.
3) Navionics 'Sonarcharts' add additional depth detail, aggregated from all the boaters who let their sounders upload to Navionics. I turn this option on when entering a shallow gunkhole. My experience in the Puget Sound has found it remarkably consistent (your mileage might vary, especially in other locations).
4) AIS integration (if you have a receiver)
A few more
notes about AIS in my boat thread.
That said, I am considering a real chart plotter one of these days...