I'm actually thinking of using both my Garmin GPS and an iPod app (maybe CoPilot), for different reasons. For turn-by-turn I'd stick with the Garmin. But sometimes I just want a sense of where I am. What town am I passin? What river is this?, etc. It's more for sightseeing and curiosity than navigation. Garmin is close to useless for that. Partly because the screen is small, partly because they are totally focused on giving directions. I'm hoping that an iPad app with the bigger screen would be better for that.
I find Garmin's lane assist less useful than you might think. For example, near me there is a freeway split where I frequently take the left side. Garmin shows 4 lanes, two going each way. Fine, but a half mile before the split there are only 3 lanes. I need to be in the leftmost lane, center isn't good enough. But lane assist doesn't tell me that. The highway signs tell me more than lane assist does, and if I'm watching the GPS instead of the signs, I get in trouble.