I use the GPS DVD included with my BMW. Previously I used a Garmin which was pretty nice. I just made sure to input the addresses prior to meeting with clients, you definitely don't want to be entering it while your driving.
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.
From what I hear the iPhone Garmin GPS app is one of the best. I haven't tried it because I have always swayed towards the Android platform and use Google Maps and Navigation. If it is easy of inputting your address: search them on Google Maps then send the address to your phone, then use your phone's Navigation system by selecting the address.
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.
I use the standard map that comes with the iPhone when I need it.
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.
I am happy with the TomTm that I bought over a year ago. Its audio has an accent but the device is very easy to use. Like another agent mentioned, if you enter addresses before showing properties, you would not do anything except click on the address. Screen is big and easy to see.