I have a friend who has an Opteka, and it is complete and utter junk. While his isn't the 40D version (it is the Nikon D80 one) the build quality is EXTREMELY poor. It is only a couple of months old, has never been abused and is falling apart.
I don't know anything about the other brands, the is is the only one I have personally seen. I don't even know if the 40D one is better, it may well be... I just know the one Opteka product I have ever had my hands on is junk.
For what it is worth.