I've read about the 15-25ft GPS error, but in practice, I really find that it's much more accurate than that. It might be a software averaging thing, where it can align several contiguous points, but when I use my Garmin 305, which is a "small" GPS unit, it overlays on GMaps consistently to under 15 feet. It rarely has me overlaid in the middle of street if I'm running on the right hand side - 25 feet error would surely put me into the street if it were consistently that much off.

I put the "track" graphic up above, and keep in mind that those track lanes are less than 5 feet wide each, and the unit NEVER has me drifting into lanes 1-3, which were blocked by a bar, over a 40 minute run. It's performed similarly in tree cover on trails and on streets when I upload to Sportracks and overlay Gmaps. At least from my actual usage experience in the past few months, it's rarely off by 15-25 feet, and if anything, closer to 5-10 feet.