The Battery: Sucks. I thought the Nexus One was power-hungry, but the Evo takes battery inefficiency to new heights of ridiculous. With some discipline and power optimization, I expect the Evo could survive a full day the office with heavy texting / moderate browsing. But just barely.
That said, the Evo's battery life isn't as bad as reviewers make it out to sound. On a practical level, it means you'll probably need to charge once a day or once every other day, depending on your usage. Keep 4G, WiFi & GPS turned off until needed. The 1 GHz processor makes enabling any one of these things take just a second or two.
The Screen: Is the best I have ever had the pleasure of using (tied with the HD2). The larger display makes the on-screen keyboard much more enjoyable to use than it is on smaller Android phones. The Evo's large screen provides one of the most pleasant long-term browsing/reading experiences I've had on a smartphone.
The Power: The Evo is a mighty phone, with 512 MB of RAM and the famed 1 GHz Snapdragon processor driving it forward. Apps launch instantly (or near to it) and multi-tasking is just as much of a breeze with the Evo as it is with the Nexus One.
Design: There is no right answer here. The Evo is either huge and cumbersome or solid-feeling and comfortable, depending on the size of your hands. I have huge mits, and the Evo fits well in my monster hands. I'd recommend going to a Sprint store in person and fondling one for a while before you decide.
4G: Sprint's 4G network is a revolution. Pages just load. After two years of wading through T-Mobile's often-laggy network, 4G is mind-blowing. Yesterday I turned on the Hotspot feature and had a friend hook into the WiFi with me. We browsed, chatted and streamed YouTube videos for several hours with little appreciable lag.
Should I Buy One? That depends on what you want out of a phone. If you travel a great deal and want a great wireless hotspot that doubles as a pretty great phone, the Evo is your best solution. If you work a desk job and just want something you can browse/chat/game with and not be seen by your network admin, the Evo is also probably your best choice.
But if you work 8-10 hour shifts at a retail / food service type place that keeps you on your feet constantly with little-to-no chances to charge up, you're better off going for a more practical device.