I purchased a cheap $40 chinese knock off from eBay, after about 5 or 6 charges, the capacity was down to 1500 mAh and after about 20 charges, it was dead completely. The battery would charge for 30-40 minutes after being unplugged by accident and immediately plugged back in, probably burned it out.
By the real thing, it's not worth buying something thats going to junk out, and wind up being money tossed in the garbage.