I bought the Garmin Nuvi 255T about two weeks ago now, and have used it to do quite a lot of long travelling in England. I chose it over a TomTom because for the same functions, it was almost a £100 cheaper, and it looked better too...
My first impressions were very positive. It looks very simple, and is easy to use. I was happy to see that you could change the mapping options, from the perspective view, to a view from above, where the map moves under the car, and another view from above where the car moves on the map (which is aligned north), which is my own setting.
Second to this the voice commands are clear, and the display is excellent, even in extremely bright weather. The night mode inverts the map, so that it doesn't glare, and this turns on automatically.
However, there are a few points I wasn't too happy about. A few times now, it has told me to turn when the road turns that way anyway, so I'm not actually turning off the road. This I found annoying, and I found the time estimations to be off, on a long journey, with no holdups worth mentioning, it was short by about 15%. The searches for amenities, or `points of interest' can be a bit hit and miss, but when you use the map, especially when out of your car in `pedestrian' mode, to find local things it is very helpful, and impressive in the scale of the coverage.
The speed camera detection is brilliant and has worked every time! The traffic detection works as well, (which has an ariel in the rather bulky car power lead), and the mapping is excellent, picking up one way systems with no problem.
I heavily recommend this sat nav.