pretty much the best helmet cam out right now is the GoPro HD.
thats what i have and i just cant get enough riding in with it

pretty much if i get on my bike i put the camera on thats how much i love it. I even used it to record while not on a bike as well and the footage came out awesome.
some of the pluses about it are.
several resolutions and it takes pretty good pics.
LOTS of different mounting options and if you run out of mounts that it comes with they are pretty cheap for new ones.
Waterproof so if your name is mike and you put your goldfish over the falls it wont die

its also waterproof for up to 180 feet so if you like to dive its still usable.
The main housing is only 40 bucks if you break it.
Quality of the video is amazing.
a few Cons are...
Its kind of easy to over tighten the screws and break your mounts. I broke a couple doing this.
It is kind of bulky compared to some of the other helmet cams so if you use it on your helmet it is acceptable to being hit by a branch or something. Or you can get the chest harness mount wich would eliminate this.
you need to buy a SD card separate of the camera itself as it does not come with one but is required.
until they come out with or you make your own the lens will make underwater quality a little less. it has to do with the curve of the lense on the housing that makes it a little blury underwater.
no view finder so you need to know your angles when its mounted or you will get home upload the footage and it might be off somewhat.
no way to zoom in or out.
thats about all i can think of right now if i think of any other pro's or Con's ill post em but for me the Pro's out weigh the Con's by a long shot. If you havent already you can look at ...
http://www.youtube.com/user/Travace
or
http://www.vimeo.com/user3887889/videos
to see some of the vids i have made with it.