The web site on the link says " The 3500MAH Extended Life Motorola Droid RAZR Battery comes with a black replacement cover with that allows full access to the phone’s functionality."
*I* interpret as the battery is thicker than stock, and requires a different back. Or worse, you take off the back and leave it off, and this cover is all that holds your battery in place. There is no picture on the site, which doesn't make me feel all warm and fuzzy either.
I don't think I'll be rushing out to buy one of these. My phone gets through a day just fine, and I bet there is fine print somewhere that says words to the affect of "open your phone and remove the battery = void your warranty". After all, Moto says the battery is not replaceable/removable. Typically that means 'keep out'.