Haven't used it myself, but the Xiaomi (http://amzn.to/1UPTQNl) is a popular GoPro clone that sells for cheap and has been reviewed surprisingly well. Low light is probably still in the GoPro's favor, but check out some YouTube comparisons -- they're not far off.
IMO, GoPro's biggest issue is usability, and it's an area where they could fight back against cheap knockoffs. GoPro cameras are frustrating to use, and editing hours of footage is not something most people enjoy doing. A commenter above suggested most owners use the GoPro a few times and then forget about it. Usability is, IMO, why.
