How good are Cycle GPS systems like memory map???

[JontyB]

I'm considering getting a cycle GPS, either the Memory Map Adventurer 2800 or Garmin Edge.

I've never used one only looked at a few in the shops.
My questions are:

1. Do you use them and how regularly
2. How good are the the stem mounting brackets.
3. Can I sit at my computer and plan a route, either on or off road and then when riding just be told turn left in 100m etc with my planned route shown on the screen and will it re route me if i stray?
4. Which one would you go for?

All comments appreciated