I picked up this unit for a 23 foot boat, because I wanted something portable.
I read all the negative reviews, and wondered what the reason could be. Should I purchase an older portable Garmin unit? After all, the 276c gets a lot of praise. After looking around, I realized that I could get a new or refurbished 640 for price comparable to what the old (and now unsupported units) now go for. Once I got the unit I realized that the fears were unfounded.
First of all, the screen is larger than the 176c/276c/376c/478. Because it's a touch screen, it's also quite easy to pan around. It's got the latest maps already built in. It's fast and accurate as well. I have no problems seeing the screen in bright daylight. It has the features you need from a gps.
I understand people's attachment to a unit that they already had, but why would you get an older, unsupported unit when you can get a newer unit that's about the same size, but has a larger viewable screen? I also picked up a set of g2 vision charts. I'm not sure they're really necessary, but they do give you point to point navigation.
At the end of the day, a gps just tells you where you are, and some units show your position on a map. If you want a transportable unit that has a decent size screen and updated software this is a good unit for a reasonable price. The built-in maps should be sufficient for most people's navigational needs.