I have a couple of the earlier HD808 cameras (one each for front & rear footage) that I bought to collect "black box" video in case of an accident. They're light and unobtrusive -- I dislike the GoPro-atop-helmet look, but after 6-9 months the HD808's batteries were good for only a small fraction of the claimed runtime -- they won't even last through a 10 minute ride. So to use them I now run USB cables to an external USB battery that I stash in a bag on the rear rack (I think that cost only a little more than buying replacement batteries and going through the hassle of soldering them in; newer versions apparently have non-soldered batteries). Videos are a little jumpy (720p 30fps in theory) and noisy (drivetrain hum on both, and IIRC wind noise in the front). The cameras don't seem waterproof at all, but they run pretty hot in summertime, so I haven't dared trying tricks like mounting them in Tic-Tac boxes to keep them dry. Even in dry weather, it's a hassle to take them in when locking up the bike, and the firmware for the cameras I have won't automatically delete old movies, so every week I'd have to pop the memory cards and delete the MOV files with a PC. So I seldom use them anymore.

As for how I mount them... the front sticks to my handlebar's headlamp with velcro. The rear is held by rubber bands to a narrow shelf I fashioned under my saddle rails with small pieces of metal and plastic. Each has a few inches of fishing monofilament tied to it as a failsafe tether. I've thought about using velcro and flexible plastic to make canopies to wrap around the sides for rain protection. Maybe if I did that and bought bigger SD cards I'd use the cameras more often. I bought them mostly to gather footage in case of a crash, but that's a rather depressing thought, which probably explains why I haven't worked out rain protection (nice as that would be for the occasional snowfall).

I think I'd be more likely to use helmet-mounted, waterproof cameras with decent batteries. You can get a pretty good deal right now on the ContourROAM version 1, which looks about perfect -- if you don't mind adding over 10 ounces to your helmet weight to get front & rear footage.

I mainly run the HD808s now if I'm in the mood to capture normal footage (and it's not raining). For instance, the anti-bike folks have been pretty loud around here lately, so I've been running the cameras to get footage of actual behavior. Naturally the motorists are pretty bad -- rolling stops, ignoring stop signs, not using turn signals (all it takes is moving yer blasted pinkie finger one inch, one time; the darn car turns off those signals for you!!!), etc. Pedestrians, too -- they'll cross against the signal whenever they think they can get away with it.