I have a Mio C520 that I bought about two weeks ago, and I love it so far. You can't touch the features in a 4.3" widescreen for the Mio's price. The receiver is very sensitive as well. Sitting here in my home office the unit has eight sats and high accuracy through my roof.
As someone else mentioned about the other Mio units, this one is also an MP3 player, a media viewer, and a bluetooth hands-free phone interface. It's also just as hackable.
The only complaints I have to date are the 2.5mm (3/32) audio connector, and the fact that you have to get out of the mapping app to change sound settings like volume or the playlist.
Speaking of playlists and how open the Mio is, one of the first things I did was write a utility to convert Windows Media Player playlists (.wpl and .m3u) to Mio's proprietary binary .m3u format. So now I can set up my music, synch a crapload of tracks to a 4 gig SD card, and use WMP's interface to create my playlists. When I'm done I run the tool to convert the playlists and validate that the list is correct and all the tracks are present on the device. Here's a link to the program for any that are interested:
http://www.modalogica.com/down...ileid=548&libraryid=36