Is it worth the investment? What type of battery life if used on average?
It depends. Are you going to be rooting it?
I got about 8-9 hours of moderate use out of mine when I first got it. I ended up rooting it and flashing some custom roms/kernels. Right now I'm running the CyanogenMod 7 nightly build from 5/25 with the Tiamat 4.0.4 sbc kernel and I use my phone about the same way, but I get a full days charge. On stock I'd get a low battery warning 8-9 hours in. Now I get a charge that lasts the whole day and when I put my phone on the charger at night it often has 30-40% of charge left on it.
If you want something closer to stock, including the Sense interface, I'd suggest something like the Fresh or Myn's Warm roms + the Tiamat kernel. Keeping at least a backup of a rom with Sense on it is needed anyways, because if you use one of the open source roms like Cyanogen you can't pull down prl updates (you have to nandroid backup cyanogen and flash to a sense rom temporarily to do so). Also spend the $2 on a program called setcpu. It allows you to create profiles for the cpu governor for doing stuff like clocking down the cpu in your phone when the screen turns off. There are some tutorials for it over on the XDA forums. Anyways, Fresh and Myn's also remove most of the crap that Sprint loads on the Evo. I was running Fresh + Tiamat a while back and got pretty close to the same battery life as I'm getting with Cyanogen right now.
Of course all this requires root so you're going to void your warranty, but rooting this phone is definitely worth it. If I were looking for a phone right now though I'd try waiting for the Evo 3D. Rumors are it's coming out at the end of the month.
There are many things to consider before buying the Evo. First, the new model will be coming out before the end of June and it will have a dual core processor, so it might be worth waiting for that. Now on to the actual question.
I bought an Evo the day it came out and I have gone back and forth about the battery life. First, the screen is huge, so it will drain the battery. However, a dark colored wallpaper will help increase the battery life and not using a "live" wallpaper will help too. Make sure to add the widgets to your home screen to turn off wifi and 4G when not in use. This helps a great deal! Make sure that background apps are not running if they are not needed. Also, many widgets on the home screens will do a background sync and kill battery life.
I bought my Evo to replace my iPhone 3GS. The battery life on average will last about 12-18 hours. I listen to music all day at work, so it will create a drain, but when just doing light browsing and texts, I can easily get 18 hours. To me, knowing the power and options I have with this phone, the battery life is a con, but one I am willing to live with. Look into the 3D before buying, but it is a great option.
(PS - you can get an extended battery from Amazon that has great reviews for around $50 and some reviewers said it gives them 48 hours of use)