The battery arrived before it's due delivery date, and it works absolutely perfectly - the battery life is a vast improvement on my old one.
Some of the reviews I've seen on Amazon for other Sony Ericsson batteries say that the battery provided was cheap knock off rather than a Sony Ericsson original. This is definitely not the case for the one I got; while it wasn't a brand new battery, I'm sure it is a nearly new (i.e. refurbished) battery, probably from a returned faulty device with a perfectly serviceable battery.