i've got a garmin 301 and can echo much of what is said on trails. i also think the nature of single track winding trails also compromise the accuracy of a GPS in wooded areas.
on the flip side, on the roads it's very handy. i have had problems in big cities (boston) with getting a signal when in a downtown area.
for the most part…it's a fun gadget…once you get used to the larger instrument on your wrist.