I have owned 2 of the official batteries, and 2 of these 'cheaper' batteries for over a year now, and I can only highly recommend these ones.
Both batteries take more or less exactly the same number of photos on a single charge, with or without flash, and I've had no problems with them at all.
The only minor issue is whilst the remaining charge shown on my camera gradually decreases for the official batteries, these batteries go from showing full to empty in about 10 shots. However as I said, it still takes the same number of photos as the official one, so it's never a problem.