We have the TomTom one, which is a rather inexpensive model. In fact, I just read that it can be had this Friday for something like $60, that is, if full-contact shopping turns you on.
Being an entry level unit, it doesn't speak street names, instead it says things like "Turn right in 200 feet" or "After 100 yards, take the highway right." Street names would be nice, but not having it hasn't been a problem either - on those few occasions I needed clarity, a quick glance at the display shows the street name clearly.
As others have said, they aren't perfect, but are helpful and nice to have. We have used ours quite a bit, including a 2000 mile round-trip we took out to Iowa and back this past summer. It is great that they automatically re-calculate if you miss an exit or a turn, and having the nearest restaurant/gas station information available with the tap of the screen is a very nice feeling.
One thing I wish it had (and it very well might, I confess I've never read the manual!) was the smarts to know about road restrictions - Such as tunnel prohibitions for propane. If we're taking the trailer on a road I know has tunnels/bridges, I do a quick sanity check online before we leave.
- Frank