My garmin has a track log of 2000 points that once full starts overwiting them - don't know if this is the same of newer models however I managed 5 hours riding and it was 99% full.
Good things are that it shows speeds altitude direction and times for each segment of the route. I should also mention that this is a serial interface model so you need either and older PC with RS232 port or a usb to serial converter. After I decided to miss a turning (ahem) it replanned across country to get me back and to be honest I was glad it did - nice scenery. Newer models will probably have the advantage of faster processors wich will speed up drawing time and route planning.
As chrisn says use the mapsource to plan your route as if you just enter the start and end address the gps will take you on the motorway when you maybe don't want to. Oh the only biggie I don't like is you cannot search by post code, You can add it as extra information but it needs road name and city to find the location (unless I'm doing something wrong) Accuracy is good for position and speed (my clocks about 2mph out) and as with all accuracy depends on how many satelites can be acquired.
It has major roads in already for the world and you upload the rest from mapsource. So long as you buy new youll be entitled to any updates. Tech support from garmin is a bit on the slow side. have a look in the google groups
http://groups.google.co.uk/group/alt...ite.gps.garmin
for any tech support issues and look for a tomtom group.