GoPros are great but expensive and a little clunky in size. The FPV RC drone racing world has given rise to a large market for smaller Hi Def cameras.
The Mobius camera has become very popular. I got one for my FPV racer and now have one as a HG camera. The video quality and definition is up there with a GoPro. The colors are more accurate, although some people may prefer the saturated color tones of the GoPro.
Reasons I like the Mobius cam.
#2 The Mobius is a Lot smaller. Less than half the size of a deck of cards. .75" thick, 1.4" wide, 2.4" long.
#3 Simple controls and you can take the memory card out without opening any case. Even changing video modes is easy.
#4 Mini USB jack in back to look at videos or charge up the battery
I got my cam from http://www.buymobius.com
A wide angle version with a slightly larger lens is available too. That works well for FPV models, but I prefer the standard lens for HG/PG use. The following is a short clip from flying this weekend mounted on the wing sprog. Select HD 1080p mode and full scree to see the detail this tiny $75 cam has.