I have one of these. I am such a huge fan of the original show (yeah, I'm a geek, sowhat?) that I had to get one.
We got ours for $159. We do NOT keep KITT's voice turned on all the time, simply because William Daniels didn't record the text-to-speech, so while he'll read freeway numbers, etc, you get no street names.
He does, however, greet me by name every time the unit is turned on. Which is, needless to say, quite awesome.
It's a good GPS. It gets us where we need to go (over the road) and has never given us WRONG directions, except for telling me that my office was on the right side of the road, when it was on the left.
Mio has not released a map update for it since I bought it last year, so I don't know how the first map update will work out.
I have no real complaints about it. Too bad they didn't have William Daniels record the TTS entries ...
EDIT: also, what's up with the cancellation of the new 'remake' of the show? I was just starting to get into it when they realized they were idiots and changed the format ... it was slowly becoming more like the format of the old show, and then it was canned.