I am an extremely heavy user of my Nexus S. I regularly use hours of display-on (2+ hours almost every day, 3+ hours many days), I sync multiple high-traffic email accounts, I live in a poor coverage spot, I am on the phone with bluetooth for my commute, and use battery-heavy applications like navigation frequently. On a typical day, this was fine, but on a substantial minority of higher use day, my phone would often run out of battery sometime in the evening. I always plug it into the charger at night.

A couple of months ago, I bought an "Extended Life Battery". It makes the phone thicker, but not unpleasantly so, and it never runs out of battery, even on extremely high use days. It's changed the way I use it too because I'm no longer hesitant to use GPS navigation or the like for fear of draining the battery. On the few occasions where I don't plug it in overnight (eg, red-eye flight) it lasts the whole next day.

Obviously, for most people, whose usage is less heavy, the thinner phone is a good trade-off, but if you're a heavy user, it's really worth it and I highly recommend it.

http://www.amazon.com/gp/product/B004YIZV6C

(On Android, you can see your screen usage by doing Settings > About phone >Battery Use, and choosing the use like Display/Calls/Maps etc. )