I have one, and for the price they're great.
Certainly not in the same league as the new GoPros, but they're silly money unless it's something you use all the time. The videos are a little cold looking in the white balance to me, but I'm sure this could be altered in post.
For me the SJ4000 is something I can mess about with and let the kids play with ( try strapping it to a 2 year old for some interesting results!) without risk of them loosing / damaging something expensive (although they're tough little cameras and I've yet to damage mine!).