Had some good opportunities to try out a few different GPS units over the last few years when mapping trails. Currently my favorite is my Garmin Edge 300 gps. Built as a cycle computer, it's tiny, mounts nicely to the stem and makes really clean and accurate tracks of my rides. Built in barometric altimeter helps to keep the elevations accurate too.
Previously I've used Garmin and Magellan handheld GPS's with much more success with the Garmin models. Magellan tech support is mostly overseas now and quite useless when it comes to helping out. Haven't had much need for Garmin tech support so I really can't say how good or bad they are although they seem to keep updates to their devices coming regularly.
Look for the x series Garmin's such as Vista or Legend HCx GPSMap 60Cx or any of the new Colorado or Oregon models which have similar antennae. Provides for very accurate tracking in steep valleys and under dense forest (like we never see any of that around here)