I have the Garmin forerunner 305. basically the 310 looks like a waterproof, more useable version of mine.
It works really well for tracking your route and monitoring performance with the HRM, the GPS chip is accurate (even in the steep sided valley at nant yr arian) and the battery life is good. (it lasted a 7 hour rat race around birmingham). I also got the cadence sensor with mine. not much use on the mtb but pretty good on the road.
Mainly its just for keeping track of my training and events, on the odd occasion I have tried to use it with memory map for navigating a route its been ok but not as good as my smartphone with mm installed.
HTH
Matt