Hitman,
I have recently bought a Garmin 800 and am really impressed with it.
If you are cycling in the UK the OS mapping is excellent and I have not had any issues with it on or off road. The only drama overall is when it rains. Mine leaked and I had to dry it out and reading the screen is difficult in the rain and especially off-road when there is mud involved. Buy screen protectors!!
I downloaded the free mapping from openmtb and found the UK street mapping okay. When I tried to plan a route in France I found it was missing lots of the minor roads so I bought the Garmin mapping for France only and it seems excellent.
When navigating on road the OS or street mapping works similarly to a TomTom in that it recognises that you are on a road and will navigate you through your route with a series of beeps and navigation screens. When you are off road it does not recognise tracks and bridleways so is a bit more difficult especially at speed.
Overall; very impressive but I would advise OS and Garmin maps.
Dogsby