I've just treated myself to a 310 (from Handtec, who are selling them "newly overhauled by Garmin" for £150 and, claiming to be good as new - I'd go along with that).
It's slightly sleeker than the 305, which for me it's replacing. Has all the same functions and similar interface. Battery life is twice as long, the virtual partner is better implemented, and there's a vibration alert on top of the beeps.
I like the older-style Garmins because the screens are big enough to read.
I think the only feature I wish the 310xt did have was a run-walk training programme, but that can probably be bodged on the 31xt if I do some research.