For all those wondering, I got one of these batteries. It is slightly larger then the HTC 2150 extended battery and slightly smaller then the Seidio 3500 battery. You can use the HTC extended cover but there will be a small gap around the sides. With the backing that comes with the battery, it makes the whole thing feel very big. The max thickness is really the same as the HTC extended case, but it is big all over instead of just over the battery.
In addition to the case being clunky, I really did not see any benefit to this battery. Even though it says 3500, it gets less life then the 2150. If you want a lot of life out of your phone, go with the 3500 battery from Seidio. It is expensive but you get what you pay for. No matter which of these three batteries you get, they are all going to add the same amount of thickness which is approximately doubling the size and weight of your phone.
If you want to increase the life of your phone and not increase the size, Seidio also makes a 1800 battery that is good. I am using the one I had from my Eris.
