I've had this running watch for several months now and I love it. It has a good size, some might say it's too big to be used as an everyday watch, but I think it looks and feels okay. The 610 has power for about 8 hours of GPS usage with a full charge, but has enough power for several weeks if used as an everyday watch with the GPS off.
I am also the owner of a 310XT and besides the much more convenient size, the biggest difference for me (I don't really need the multi sports functionality) is the touch screen on the 610. Of course it is fair to wonder why the hell you need a touchscreen on a running watch, but it turns out there is a damn good reason. It is incredibly intuitive and easy to use. The 310XT has a user interface that seems to have been designed by engineers for engineers. The 610 is so easy to use that it's a joy. You have to use either a nail or press fairly hard with your finger, but once you've gotten used to that, it works like a charm.
It has all the functionality you need for running and obviously all data can be analyzed later using the Garmin Connect web application. All transfers are done wirelessly thru the ANT USB stick. The pulse belt is a soft strap model and I like it much better than the old hard strap version.
A new function is the Virtual Racer. The old mode Virtual Partner is however still there. Virtual Partner keeps a set pace throughout the entire course, while Virtual Racer is based on a previous run, meaning it makes it possible to run against a previous "ghost", memorizing your position at a given point of the course. Very useful indeed to pace yourself for a new course record.
GPS reception is generally very good, but like with all Garmin GPS training devices it varies exactly how fast you'll get a satellite lock. Sometimes it only takes 20 seconds, but the day after it may take several minutes. I wish they would build in some assisted GPS, but most of the time it works pretty fast. I've customized a screen to be able to monitor the GPS accuracy while waiting to begin a run. It is quite important not to start running until the watch is down to an accuracy of about 10 meters to ensure perfect GPS tracking.
The 610 offers all I need from a running watch (actually it also has a bicycle mode), and I'm glad I decided to move on from the 310XT which I never fell in love with at all due to the inconvenient user interface. The garmin 610 is rather pricy but I got a very good deal, so I can't complain.