I have the 310xt, I was at first worried that it would seem to bulky as I have fairly small wrists. Its actually really comfortable and I don't notice it anymore than I would my regular watch.

I like the fact that its waterproof so I can wear it for the swim and don't have to worry about it getting wet. I do use it mostly for running and swimming as I have another bike computer.

I find the HR useful to make sure I am working as hard as I should be (especially on the spin bike) or that I am not working too hard when trying to do recovery runs.