Here's some advice from someone with a Hero.
My phone has the same 1500mAh battery in it. I was getting over 24 hours on a single charge on it, but missed dearly the 2-3 days with my LG dare. I got the 3500mAh battery. I loved it. I got over 2 days with it, and it still fit in my generic belt clip holster I bought at Best Buy. I stopped using the holster (no more dress clothes for a bit) and kept it in my pocket. I didn't care about the width in my pocket, but the aftermarket back is cheap and has a crappy fit and would come off in my pocket. I've since gone back to stock, and keep my 3500mAh around for an emergency. I've also built a custom windshield mount (awesome btw, for under $15... suspect could easily be done for the evo shortly after release) and it fits only the stock battery. I'm pretty happy with what I get out of the 1500mAh battery, and rarely push my phone to do well more than it should without knowing I will have a USB port handy.