With my Holux M1200 (which I also intend to use with the N900 instead of its built-in GPS) the accuracy seems to be quite good. I have lots of tracks in maemo mapper, from driving hundreds of kilometers. The tracks consistently show which side of the road I'm driving on (from driving one way, then back again). The inaccuracy I see some places are all caused by a fixed sideways error in one direction which only affects satellite images (not maps), for some reason.

With my older, less sensitive etc. GPS I would often see the location jitter all over the place sometimes, particularly inside cities. There's very little of that with the M1200.