Sorry to hear it isn't enough for you. Let us know if you end up going for something else. I would be especially interested to see if you get something else that fits under the stock battery cover, how it works out for you compared to this one.
Mine added just enough juice to keep me happy. I did try one of the generic xtra large capacity batteries, but didn't like the look/feel, the signal sucked due to a really bad antenna in the door that came with it and it really didn't add enough life for the size.
Some people seem to really like the feel of the phone with an extended battery. Just make sure to get one of the ones from HTC.