9/4/2011 UPDATE: I have had this battery for over a year now, and it is still going strong. It isn't quite as beastly as when I first got it, but I still can go a full day of hard usage and not kill it. Every once in a while, I get a runaway app that will start burning it down, but it's obvious when that happens, and I've been able to stop it. I still recommend this battery for everybody with an evo, and encourage you to send yours back if you have any problems. I absolutely love my phone, and it's because of this battery.
This battery is ridiculous! I took it off the charger 30 hours ago and it still has 70% of the battery (though that is with normal use - 4g off etc.)
My average wake-time in a day is 18 hours, so I wanted a battery that would be strong enough to take a beating and last all day. That was my main reason NOT to get an iphone (other than their terrible network.) I didn't want my music/internet/video games using the same battery my phone does. I've been stranded before with a dead battery and vowed never again would I have all those devices in one until they made a battery that could handle it. This thing does all that in spades.
Turn on 4g/gps/bluetooth/wifi, the flashlight, have pandora playing, and watch a movie at the same time FOR HOURS AND IT WON'T DIE...not that you would want to do that. I did while trying to drain the battery for conditioning before I learned that you don't need to "condition" these batteries. More on that later...
The battery does add some bulk and weight to the phone, but it feels good in my hands, especially when texting in landscape mode...like I'm holding a video game controller or something. Plus, as many people have stated, the camera lens is totally protected by the little shelf of the battery sticking out. Despite what another reviewer pointed out, the battery does not affect the pictures at all. If it does, you have a defective product and you should return it.
The evo 4g is a beast, and if you want to be able to treat it like one, you really need this battery.
Update: it's now been 48 hours and the battery is still holding at 40%. This has been with pretty moderate use (plenty of phone calls, texting, browsing, downloading, emailing, and gaming,) but with 4g, gps and bluetooth off. I have had wifi on the whole time though.
Somebody left a comment about these lithium batteries not needing to be drained all the way down (in fact, that's supposed to be bad for them) so I did some research and they were right. It's moot though, because it's not really possible to drain them all the way down. Also, as the instructions will tell you, LET THE BATTERY CHARGE FOR A FEW HOURS AFTER IT SAYS IT'S FULL. I put it on the charger with 10-20% left in it, and it charged to "green" in a few hours. So, if you're sleeping ~6 hours a night, this should do ya fine. I got some tips on battery care from the web. Amazon won't let me post the link, but if you're still not sure, just google lithium battery care or something like that.
Simple Guidelines
* Avoid frequent full discharges because this puts additional strain on the battery. Several partial discharges with frequent recharges are better for lithium-ion than one deep one. Recharging a partially charged lithium-ion does not cause harm because there is no memory. (In this respect, lithium-ion differs from nickel-based batteries.)
* Keep the lithium-ion battery cool. Avoid a hot car. For prolonged storage, keep the battery at a 40% charge level.
* Avoid purchasing spare lithium-ion batteries for later use. Observe manufacturing dates. Do not buy old stock, even if sold at clearance prices.
* If you have a spare lithium-ion battery, use one to the fullest and keep the other cool by placing it in the refrigerator. Do not freeze the battery. For best results, store the battery at 40% state-of-charge.