I got it two weeks ago and it hasn't let me down, I go through a 5 am work day till 2, and then use it most of the day until about 9 or 10 and I still have about 25% battery left. I am a heavy user on my phone so I'm glad I have this battery too.
I have had one since the week they were released by Seidio, and I cannot imagine trying to use the Evo with the stock battery now. It is just so nice to know that I can use the phone very heavily all day long and not wind up with it dead at a bad time at the end of the day possibly.
The great thing is that I have often gone entire weekends at home without plugging my phone in at all - it is relatively light use when at home on wifi the whole time, but 50+ hours (longest I noted was 62 hours) with the phone continuously on and unplugged is awesome.
Heavy use work days, 60 minutes of calls, often checking emails and reading news articles every 15 minutes, listening to audio books or music while doing other work, and maybe even half an hour playing Angry Birds thrown in, and the phone handles a 15-16 hour day with 50% charge left sometimes.
That battery is what makes the Evo capable of being awesome for me - the stock battery dies way too fast, making you plug the phone all the time or dramatically change your behavior to accomodate the battery.
I broke one of the tabs on the case back for this battery so it no longer closes tightly. I'd like to buy just a new back but it doesn't appear they are available. I've emailed Seidio a couple of times now inquiring on availability but aside from automated responses with a ticket number I have never received a reply from them (really irritating - what is their support contact for?). I've considered one of the other brands available but it appears that the thickness of extended-capacity batteries varies. I don't want to shell out for a new battery just to replace the back.