The OLYMPUS Li.40B Compatible Battery (3.7V 550mAh Li-Ion) is only 550mAh compared to the 750mAh that comes with the Olympus Cameras. I tried both of these batteries out and they took the same amount of pictures, (about 260) over the course of the entire day. The only difference is the first one ran out just a bit sooner as expected.
For 10 dollars its a great deal, but not perfect for a replacement battery. This battery works best as a backup.