On paper the GoPro Hero3 is very impressive, with a good number of video modes, including some potentially very interesting ones. Read this article for a good overview of them. It also has a built-in WiFi interface that works great with my Android smartphone. With the included housing it can take almost anything.
The form factor is in my opinion poor compared to that of the Contour Roam, which could be mounted just about anywhere without problems. The only situation where the GoPro form factor is better is for the chest harness. I also liked the Contour Roam weather proofing better, since it could take most ordinary situations like rain and water down to one meter without an extra housing. With the GoPro the extra housing is in practice always necessary and it does affect the video sharpness at least marginally.
So how good it is? Some systematic tests by knowledgeable people show that it actually is pretty good, with the video quality showing good resolution (though still far from the nominal resolutions) and color rendition.
My first impressions below are non-scientific and geared towards my own intended use, so take them for what they are. (Note that I'm using the 25 fps modes mainly, since I have some ambitions outside pure internet use. Faster frame rates are generally easier to use).
As can be seen from the video, the fisheye effect is pretty strong, which in my opinion is the biggest drawback. It can be dealt with in different ways, but I would prefer a more straight image. In some situations the fisheye effect won't be noticeable, in other situations the image can be defished with acceptable quality, especially from 2.7K source, and in some cases the medium FOV could be used to lessen the effect. Of course, there might be situations where the fisheye effect is desirable, but I generally prefer trees to be as straight as they are in the real world.
The GoPro is nowadays very easy to use and you can actually frame it fairly accurately with a smartphone. The Black Edition also includes a WiFi remote, but I don't see myself using it much when the smartphone interface is so much better.
The GoPro contains fairly good timelapse functionality, but in my opinion it is limited by the auto exposure and fisheye lens. See the video containing a short timelapse shot with the GoPro. The exposure might have been better with the spot light meter on, but I think I will do my timelapses mostly on the Canon S90, since it has full manual control and a lot better optics. For self documenting I do see some possible use of the timelapse functionality, though.
The 4K mode was a disappointment, but I guess it is more a marketing gimmick than anything else. With 15 fps it was never good enough for motion, but some so called reviews hinted that it would be a good way to get a fast frame rate for 8 megapixel stills. I tried it and the stills are just video frame grabs with similar quality. The resolution and colors are nowhere near real 8 megapixel photos. Here is an example, with the full size version being here.
What more to say? The battery life is poor and on my latest outing I got about 12 minutes of use of it. Granted, it was cold and the GoPro was in ambient temperature all the time, but my Panasonic GH2 was in the same temperature and the first battery lasted one and a half day with lots of pictures and about 30 minutes of video. I still haven't been able to get a GoPro spare battery, they seem to be out of stock at least here in Finland.
Finally, I think the video below shows the amount of POV angles I'm intending to use. Maybe I could have a few more, but I'm not going to overuse the them. Most of the video is shot with the Panasonic GH2, but the timelapse and the POV shots were done with the GoPro.