I've just bought one, and I'm genuinely surprised with the quality of it! I bought it off here for £30, expecting it to be very plasticy and a bit delicate, but it feels really well made, and very sturdy. I couldn't compare how it feels to the genuine canon one, but it feels just as well made as the body itself, so I doubt it's much worse! In fact, I actually prefer the textured grippy bits on the 3rd party grip to on the camera, they feel better quality!
It's one by 'Linkdelight', who have a website and sell through ebay I think. I haven't really used it get, but I'm very impressed so far!