When I have more time I'll write up my review of the Garmin 610 coming from my standpoint. Someone who runs 80-100+ miles per week when training for a marathon.

Just a few examples of the "accuracy" of the Garmin 610. In the pictues with green and red, the green is my Garmin 305 and the red is my Garmin 610. One of the other pictures is from another runner using a different Garmin 610.

I've never seen these "jaggies" in my Garmin 305 data.

The Garmin 610 measured 12.40 on my run. The Garmin 305 measured 12.52. When looking at the actual GPX data, the Garmin 610 GPX path is actually 12.69 while the 305 is 12.57. So, the 610's actual GPX path is 0.29 miles off from what the watch state, while the 305 is off by 0.05.

Clearly, those little jaggies (or big sometimes as shown below) are screwing with the GPS distance/pace measurement of the Garmin 310. Even the Boston Marathon has a jaggie.