I picked up the Garmin 310 XT from TrySports a little over a week ago and spent the last week putting it through it paces. So far my experiences have all been positive.
Syncing the 310 with the Heart Monitor and the Garmin Cadence was very painless. As soon as I got within 5 meters, the 310 picked up the signal and paired with each device.
Transferring the data to the device is not totally painless but not extremely difficult either. To obtain the software, it does required download from Garmin support and installation of a client ANT program. Then, the 301 XT has to be paired with the USB device. But once this hurdle is overcome, the rest is all downhill from that point.
Leave your Garmin 310 on after your workout and bring it within 5 meters of the USB device will cause the data transfer to automatically start. First, the data will be sent into the Training center and if needed, you could push it up to Garmin Connect. I definitely see this as an improvement over the 305 which had to be physically attached to your PC
There are some other differences between the 305 and 310. Starting, stopping, alerts will cause the Garmin to vibrate. At first, I was a little irritated by it, but as I used it more, it started to grow on me. A little vibration gives me an extra verification that I have started or stopped my session.
Another difference is how the history is maintained. The 305 has a history of running and biking, but on the 310 workouts are described as activities. And, it only appears as an activity once the Garmin is reset via the lap button. (Honestly, I am still learning on this one so there maybe another way to see it without resetting it)
The other feature that I am starting like is the main display scrolling. Being able to scroll various attributes during the current session is really nice.
The charging of the device works a little differently from the 305. There is a clamp that has contacts to push power into the battery. According the documentation, I can get roughly 20 hours of battery life. So far, I have not preformed a longevity test of the battery, but at some point, I am sure it will happen.
The last item that I want to point out is the booting process. The 310 boot process is much faster than the 305. So far, within 30 seconds of turning on the Garmin, the display comes up. The 305 could take several minutes before it booted to the normal display and longer if it were a cloudy day.
In my opinion, the 310 is a nice upgrade over the 305.
Thoughts from the Cool Down Runner.