ok, having owned the hero 2, and done extensive studies on the newer 3/3+, I can tell you quite a lot about them, since I've developed a small obsession with them.
first off, it really depends on how you want to use it. for capturing general b-roll footage or action shots that don't require sound/dialogue, they're the best. i can't tell you what a difference it made in my videos just being able to capture video in/under water. also, having a cheaper camera that will go places your big expensive camera won't is a pretty big deal.
yeah, the battery life isn't phenomenal, but you'll get an hour or so out of one battery. the good thing is that there's a great set of generic batteries for the gopro here:
http://www.amazon.com/dp/B00AAIPT76/
you get two batteries and a charger for the same price as one 'branded' gopro battery (about 20 bucks), and they work as good if not better than the original batteries.
huge drawback: no lcd screen. that makes it incredibly hard and/or impossible to line up your shots. i never had one and just guessed and my shots were more often than not at a weird angle. getting the lcd backpack is
highly recommended for the hero 2.
the hero 3 has built in wifi, so you can pair it with your phone/tablet (definitely) and laptop (i think?) and use the gopro app to view a live feed of what the camera is seeing/recording. works pretty well from what i've seen, though there's a bit of delay in the feed. also eats up more battery. there's an lcd backpack for the hero 3 as well, but you could probably just get away with the app if you have a smartphone/tablet.
the accessories get expensive. and believe me, you're going to want the accessories. lcd backpacks, chest mounts, head mounts, handle bar mounts, suction cup mounts, sticky mounts, extension poles, etc, etc. I recommend getting the 'floaty' back door if you're going to be using it in water. this will keep you from losing your camera to the bottom of the ocean (makes it float).
to get an idea of the accessories you want, watch a bunch of the videos on the gopro youtube channel. keep asking yourself, "how are they getting that shot?" is it a gopro on a pole? head mount? etc. also, keep in mind part of the reason gopro videos look so incredible is not just because of the camera, but also because of camera jibs, sliders, stabilizers, aerial drones, and an team of color correctors in post making that shit look dank. you might want to look up some amateur gopro vids to see what it looks like filming out of the box.
one last thing, keep the camera and waterproof housing in a bag or case of some kind. getting a scratch on the lens fucking spoils everything. i got a minor scratch in the lens area of the plastic waterproof housing, and it started showing up in every one of my shots, even underwater, which was a pain. also, when using it underwater, make sure to wipe off the plastic area in front of the lens between each shot, otherwise you have a good chance of having a water droplet right in the middle of your shot blocking everything.
anyways, that's enough for now. let me know if you have any other questions.