I just bought one (hero 3 black edition) as Im planning for a gap year and have a scuba diving holiday next month.
Obviously as a big kid I've been trying it out on anything and everything and like others say its great to have such a versatile, compact and tough camera!
Despite seeing countless underwater clips on YouTube I had to try it before I trusted it so dipped it in my local stream. It still works and the flat port housing is definitely an improvement.
I've recently been using it in clubs along with my event photography so can give some comparison to the iPhone 4S in that regard.
The auto ISO is impressive and gives a crystal clear image when there is enough light. In lower light though the grain is quite noticeable and I'm torn between this and the iPhone now due to the great low light performance on the iPhone and better battery life. Having said that, you can get much better camera angles with the go pro and have the freedom to swoop it up, down and everywhere
It definitely trumps the iPhone and any camera phone on sound though - when used with the dive housing there is basically no low end distortion in loud environments. High end is a bit tinny but i prefer that to distorted bass ruining the clip!
If you want to get an idea of what it looks like in this environment check out my most recent video on YouTube. It was shot behind the Dj booth with varying light conditions. 1080p 30fps. ( medium view mode i think). The sound at the very end of the clip is straight from the go pro untouched.
Www.youtube.com/MisterSteveOfficial
Clip is 'mark sherry - trailblazer (trance nation)'
For comparison the other clip 'my love' was shot on the iPhone although is a shaky video.
Hope this helps. Viva La GoPro!