Just about 2 months with my Galaxy S II and so far no major problems. Still an awesome phone. It's already taken a few tumbles, but fortunately it's been well protected by the case and screen protector, so no damage at all.
Battery life is pretty good, and I can get through pretty much the whole day on a single charge and still be over 50% battery. Heavy internet use or being stuck in an area with poor signal or data will suck the battery dry pretty quickly though. Spare batteries FTW.
It also doesn't seem quite so big in my hand anymore now that I've been using it for a while.
One annoyance I've run into is that if you drain the battery low enough to trigger Power saving mode, all the things that Power saving mode turns off stay off. For example, display brightness goes to minimum and stays there. Sync turns off and stays off. Screen timeout goes to the shortest setting (15s) and stays there. So once you manage to get your phone charged back up, you need to go in and re-enable or reset things back to your preferred settings. It's a somewhat minor annoyance until you start wondering why you're not getting any email notifications, or why you can't see the screen when you walk outside anymore. Then you realize the Auto-sync has been turned off and screen brightness has changed from auto to minimum.
The things that Power saving mode resets or disables can be changed, but if you start telling it not to disable things, it kind of defeats the purpose of Power saving mode.