There are several poor reviews on amazon for instance and if you google some more, you'll find many complains about the watch's ability to follow a course at speed (the screen takes too much time to redraw and eventually the soft crashes).

I've used the 910xt's course mapping extensively while hiking -- it has actually impacted my behaviour significantly and I would warmly recommend GPS watches such as the 910xt over handheld units for hiking. Didn't do it for me on a bike, though. Outright dangerous on the wrist, and even mounted on handlebars, diminutive screen and unsuitable interface made me purchase an Edge Touring instead. Edge Touring doesn't support ANT+, so I've used the 910xt on my bike to track speed/cadence+HR. The battery lasts an eternity (I'd say more than 24h)