Rob's got a lot of good comments. These apply to most action cams, not just GoPros. I have extensive experience with GoPros, so I can speak to a few tips I've learned:
1) the sound quality is not that good, in fact it stinks in my opinion. Bad sound and you will have a bad video.
I agree that good audio makes a great video. Sound quality only sucks on a GoPro when you're using a waterproof case. If you use the frame or an open-back, you'll get much better audio. Check out the audio on this video, all done with the GoPro, without the waterproof housing.
15:11 especially shows how great these mics are (with some added compression in post).
2) battery life is minimal at best. There are a number of hikers each year who find this out and end up cutting the amount of footage they capture short to maximize battery life. I had a friend who took off on the AT last year and sent his GoPro back at the first chance.
If you're filming in 4k or 1080p, I agree. If you're filming in 720p or 480p, you'll get a ton more battery life. Fortunately you can control the quality/battery life balance.
For HD, you'll need multiple batteries, or a big one like a Brunton, or an external battery to charge them every night, or a solar panel charger. I have all of these and I use them all a lot.
3) and finally, accessories are expensive. Whether it is extra batteries, mounts or anything else it all seems a bit pricey.
Agreed, if you buy them from GoPro. Fortunately GoPros are so popular that there are tons of companies making mounts, batteries, and accessories for them. Amazon is a fantastic place to find replacement batteries, cases, housings, mounts, etc. But I agree, if you're buying from GoPro, it's going to be super expensive. That being said, I feel GoPro's quality is unmatched, so at least you're getting quality products when you buy from them.
A few more things I love about GoPros:
1) portable.
2) extremely durable. I have dropped mine countless times. It's even been run over by a big jeep loaded with a weekend's worth of camping gear, and nothing broke
3) it's easy to find mounts all over the place. If I'm traveling, I can find extra gopro mounts at walmart, bestbuy, sporting goods stores, climbing stores, you name it.
4) mounting solutions are awesome. The design of the sticky mounts, clips, and the little barrel adjuster is foolproof and does a darn good job. We all know that different angles make for more exciting video, and GoPros (or any wearable action cam) allow you to position the camera in so many different locations
5) wifi connectivity. This is huge. By being able to control my camera with my phone is huge. I wish my DSLR had this (for some stupid reason it's disabled in video mode on the 6D). It's a great viewfinder, big menu, and really comes in handy.
6) picture quality is insane. Many motion pictures are using GoPros for some of their sequences.
Action cams have their limitations though, and you should be aware of them.
- most don't have a screen, so you might struggle to know what's in frame. That's why the super wide angle of the GoPro is so fun, you almost always capture everything\
- no zoom
- photos aren't as good as with a good camera
- limited in use. If you're not going to be wearing it, you'll probably prefer a regular camera.
Hope that helps.