I just got this program "SportTracks" which aligns your GPS data with Google satellite imagery.

This is how accurate the Garmin 305 is during my track workout. There was a "blocker bar" across the inside 3 lanes at various parts of the track to prevent runners from over-using the inside 3 lanes, so I was forced to run pretty much on the outside lines. I did at least one warmup lap on the most outer lane, then moved into the middle lanes as I sped up and passed people. I also took one "clothing" removal stop where I headed into the bleachers on the straightaway for about 10 seconds to ditch my extra jacket.

You can see EVERY single one of these moves (including the jacket stop!) aligned on the map below. I did no manual editing to this mapped course - it was just pulled directly from my watch and sync'd with Gmaps via SportTracks.

VERY VERY accurate, in my opinion, and the Garmin is known to perform WORST on curvy courses like this one. In fact, the track is probably one of the LEAST accurate courses to use the Garmin GPS (due to undersampling on the curves), yet it's obviously stil very, very good. Take in on the roads, and it should be near-perfect.