I have one and have used it on a number of dives.
PROS: Good images, both for camera and video, especially with adequate lighting.
CONS:
1. This is the MAJOR problem and from everyone I have spoken to who has had one or knows someone that does they have encountered the same issue. You MUST make sure you thoroughly clean the mask prior to diving or it WILL fog up on you. I have cleaned it a dozen times and it still fogs up on me if I leave it in the sun for a minute before diving. I try to keep it in the water bucket until I'm about to enter the water and then I don't have a fogging issue. Otherwise all bets are off.
2. The lens angle is very small so practice in a pool or outside to make sure you know how to line up the angles before using. I missed some spectacular video because I chopped off my target when viewing it from my goggle. It is best with distance shots, the closer you get the more likely you will miss some of the target on your video.
3. battery life is quite short. I use lithium batteries and I got about 3 hours of video before I had to change batteries. I think rechargeables would be a lot less. Regular batteries seemed to work for about an hour or so. I have replaced them with lithium batteries again so I will let you know if they last any longer this time. So far I've done about 2+ hours on the lithium. Be safe and carry spares in your scuba bag.
4. Since the camera is attached to your head you really have to discipline yourself to be steady when diving. I have gotten better at this but it takes some getting used to when diving and videoing.
5. It can be difficult to be sure you are videoing something. Often I though the video was running when I was targeting a shark or turtle or what not but then I found that it was off when I returned to the boat (not the best time to figure it out). This has caused me to resort to starting the video before I dive and running it the whole time while down there. This tends to waste memory (only 2GB) so either you need to get a card with more memory, really pay attention to make sure the thing is running when trying to video, or bring a laptop with you so you can save each dive on it to preserve memory space (my method).
With all that being said it can work and produce memorable videos and photos. You just have to be aware of what you are doing and practice, practice, PRACTICE!
I am still new to the whole videography, as my videos will show (and which keeps me from posting the horrible ones I've edited on this site), so those of you with more experience might find the Cons I've listed to be applicable only to novices like myself. I do have a great 5 second video of a moray eel that once I edit I will post. Unfortunately the camera was off when I had the opportunity to shoot a lionfish being eaten by a spotted moray eel (which coiled itself around the lion fish before devouring it) but I'm hoping to recoup that loss on my next dive trip with another great photo op.
Anyway, if you get one best of luck and lets see what you shot!