I still think the cheap batteries on ebay are the best deal. Will they last as long as the OEM? Probably not. But if you use the OEM as your main battery, and the cheapo just as a backup when the OEM one fades, they're probably good enough. And at $10 for 2 batteries and a charger it's hard to complain.
As an FYI, these cheap batteries seem pretty good. I put one (fully charged) into my EVO 10 hours ago. It's been a pretty typical day for me: a few brief phone calls, some light web surfing, a ton of emails on EAS, and half a ton on Gmail, no text messages, and a bunch of calendar alarms. Both emails are on real time updates. Screen brightness is set to auto.
Done a small amount of playing with the phone. downloaded a couple of apps, and have an active wallpaper. I've not used 4G or wifi, but have had BT and the GPS radio on all day. I have weather bug set to update hourly.
After 10 hours, I'm at 50% battery left. At 2 batteries and a charger for $10, I think I got my money's worth!
Edit: I'm now at 15% battery left, after nearly 15 hours (14:53), but the Charge Now warning still hasn't popped up. I'm certainly not the hardest user of this phone, but I'd say this lasts about as long as my Palm Treo 755 did. I can't complain about this at all. Oh, there went my battery warning, at 14:55.
As a counterpoint, yesterday I kept 4G on a good part of the day, used Sprint Navigation for a total of about an hour, did a lot more playing with the phone, and downloaded more apps. I got somewhere around 4 hours off a battery. 4G and GPS navigation will really kill the battery.