I used one of the Navman icn650 versions of this device last year driving through France and found it excellent. If you can get it for £150 it is a real bargain, IMHO.
I disagree with the issue of the lack of battery - I had a TomTom Go 700 for a short while and thought it would be better because it wouldn't need to always be plugged in when in the car. This is not really how it works in real life - (depending on how you use the thing). The problem is the battery will discharge even when the device is off - it seems to be in a sleep mode. So, if you have not used it for a week the battery will probably be flat and you will need to plug it in in the car (or at home). I found the Navman interface from the map graphics to the menus and preference possibilities far superior to any other device I have seen - in fact I think the TomTom maps look quite tatty compared to the Navman/Sony.
Only real gripes are the fairly large size and the lack of Macintosh support offered by either Navman or Sony.