We also have a Garmin, and it's fine. Garmin & Tom Tom seem to be the main two brands, and each of them has a fairly wide range to choose from, going from basic ones that just tell you where to go to more expensive models that take into account current local traffic conditions when giving you directions. The main difference seems to be in the way you pay for map updates.
With the Garmins, you can pay to update your map to the most current map for £49.99, or you can pay for unlimited map updates (for that one Garmin device) for £99.99.
With Tom Tom, on the other hand, you can pay to update your map to the most current map for £49.95, or you can get a subscription for up to 4 map updates per year for £32/year (Tom Tom people, feel free to correct me if I got that wrong; I don't have one, so I'm just reading it off the website).
We haven't updated our Garmin since we bought it in 2008, and while we occasionally run into map discrepancies (mostly on bigger A roads where a roundabout has been replaced by an interchange with exits), having 2 year old maps hasn't caused us any trouble (aside from my geek nature wanting to have the latest maps - haha). We'll probably update the maps for £49 eventually.