If you're going to alter the thin profile of the HTC EVO by using an extended battery that requires a back cover to accommodate its added girth, then I think you may as well go whole hog and get the Mugen Power 3200mAh battery or Seidio 3500mAh battery (with or without the Innocase Extended case). But if you're not quite ready for that sort of commitment and you've only got $20 to spare, then check out this 2200mAh battery and replacement door instead.
It's only $19.95—cheap batteries aren't always bad—so it's quite the deal compared to the $97 Mugen or $70 Seidio batteries. The back cover doesn't look too bad either. It's not as nice as the OEM door, of course, but I wouldn't call it hideous.
I don't know if 2200mAh is worth fattening up your EVO for, especially considering the slim 1750mAh and 1800mAh batteries that are the same size as the stock battery, but it only takes $20 to find out for yourself.[Amazon]