Make sure you get a Garmin with a USB cable that way you can update it online as streets, roads and highways may change, especially with new development.
Personally, I use both a GPS and my Thunderbolt. I hate all the mounting and unmounting so I just leave the GPS there in it's mount (until I am done for the day), which also acts as a great bluetooth speaker.
However, I preview showings so that I don't have to use a GPS to know where to go. In my never humble opinion, using a GPS when the client is present is a sign of weakness - it tells your client that you don't know the area you're selling.
For safety reasons, we have maintained using our Garmin system...bigger screen, easier to use key board, and easier directions to follow.
Until the iphone can demonstrate equal service, I'm affraid the Garmin wins "pole position,"