I had the Opteka grip for my T1i and I loved it. So, when I upgraded to the 5DMKII, I had no worries about getting the Opteka grip for that camera. However, once I got it, I noticed that it felt more cheaply made (even versus the T1i one!) and I was especially worried because I use a R strap that would connect directly to the grip. I'm returning it and I'm now considering spending the insane amount of money to get the Canon one. I'm just too concerned that without good build quality, it'll break apart while I'm walking around, since all the weight will be on that grip!
Function-wise, the Opteka grip seemed to work fine.