If you want a good battery, buy one from Newertech.
Aside from that, I've had mixed luck with cheap Chinese batteries. Some have been DOA or nearly so. I've had some that wouldn't charge, or might last 1-2 cycles. I've had some that fit VERY poorly, sometimes to the point where I had to use shims to keep the contacts in place. Some will randomly cut out. I've never had one live up to its actual rating.
My most recent one(last week) was for a first generation Macbook Air. It seems to work okay, but there again the life seems short.