No idea about the lakes or the US but i have found the local maps
to be incredibly useful.
Great for quick planning in the pub.
Fab for quick reference on the water or coming into tight moorings. (So
much easier than scrambling around with paper charts in a tight cockpit)
Just make sure you have done your research on paper charts and coastal pilot.
The great thing is that the charts are locally stored so if you have no
signal they still work. This happens where I sail frequently.
Easily the best app I have bought.
I recently purchased iNavX, and have been very happy. It is a costly app, $ 49.99, but well worth it. You will also have to purchase the appropriate map, $10. This is a nice thing to have on a dark night, sure beats running into an island!