Mine are still going strong. I can get two full days of average use now that I'm rooted and running a custom kernel and rom. The stock battery now gets me through the day no problem, I should have rooted the day I got the Evo.
But for days when I use my phone for work, which means calls plus internet, navigation and camera for several hours total, I definitely need the extended battery, and often end up charging for an hour or so by evening to prevent the battery from dropping too low. I'm pretty sure they don't hold as much charge as when they were new. But I already feel I've gotten my money's worth.
There is a seller on ebay that has the Sprint version extended battery door, in black or white for $10 shipped. Mine arrived, great fit with the Chinese "3500mah" batteries, and a better design. Now I can dock my phone with the extended battery still installed. And the phone feels less bulky.