Well, the so called "knock-off's" especially the SJCAM branded ones, are supposed to be pretty good. Well, good enough to make a pretty large dent in GoPro's share price anyway. Dependant upon exactly what your usage will be, you'll probably be fine with a SJCAM.
Here's a quote from an online comparison review (note: the comparison here is specifically the GoPro Hero - the budget version, vs the SJCAM 4000 - so the closest price equivalence). If you want to spend a lot more, then clearly the GoPro Hero 4 is the king.
"The SJCAM SJ 4000, in respect to the GoPro Hero, is a far superior camera at a price far below the cost of the GoPro Hero. For around $70 you can get full HD 1080p videos, 12 mega pixel still photos, a 170 degree lens, a built in 1.5 inch LCD screen to playback your videos, several mounts, and many other features not included in in the $120.00 Go Pro Hero. In terms of quality the SJCAM SJ 4000 is comparable to GoPros last camera series the GoPro Hero 3 and GoPro Hero 3 silver. Considering the SJCAM SJ 4000 is one fourth the cost of the GoPro Hero 3, it’s safe to say when it comes to value the SJCAM SJ 4000 is your best bet."