I have the 610 but coming from the older non-GPS model with footpod, the pace reading does take some getting used to. The old footpod was not 100% accurate, but it seemed to correlate more closely with what you were doing - i.e. speed up, the pace goes up, but the GPS does have some lag and can do strange things going round sharp corners. You can bump up the sampling to 1-second (from 3) on the 610 but to be honest, I couldn't see much of a difference. I've actually set up a page on my watch to show the last lap pace, and now set the auto-lap to every half mile and use this to try and adjust pace rather than constantly glancing at the varying instant pace. Tempo runs were getting too stressful!
As a watch I would say it's been a good purchase. The display is very big and clear and the customization options for the display are excellent (see last lap pace example above). The option to have the watch vibrate to alert you is also great.
I use the intervals functionality and this works well. Unfortunately, I've not found as much use for the Custom Workout feature. This was the main reason I went for the 610, thinking I could program in the Daniels workouts at my specific paces (8:20 easy for 2 miles, 3x6:30 tempos etc.) but in reality, this doesn't work because 1) the pace reading fluctuates too much (lots of spurious speed up/slow down alerts that get annoying) and 2) you realize that you rarely finish exactly on 2 miles easy before progressing to the quality portion of the workout due to hills, traffic, etc.