I recently downloaded the official Sony Power Saver app, and activate it while at work. I can now get about 20 hours usage from one charge. Here's a link: http://dl.dropbox.com/u/19922917/powersavemode.apk
I suppose on average i can get a good 20hrs thats with gps, email, twitter sync, no facebook sync, watch a few mins of tv during lunch, at the end of the month when no credit i turn off the data and get an extra 5hrs or so, its not too bad, i see some lads peremently with GalaxyS II and iphones (sorry for bad language) coinstantly on charge,
I usually come home from school around 5pm with 50 -70% battery. During the day I play some games, browsing, texting, calling, playing music, both local files and youtube, downloading stuff from market and use other apps. Always 3g and GPS on. However I use juice defender so 3g is off when the screen is off. When I come home I use it for some browsing, and playing movies. Often til midnight. So that's 7am til midnight with average - heavy usage.
Today I had nothing to do really, thunder and rain outside etc... Used my arc s constantly for five hours with screen on max brightness, music on almost all the time, except from when I was streaming hq videos from youtube via WiFi. WiFi and sync was on update all the time. While I was listening to music I was also browsing the internet. Basically, I used the phone to the MAX. Edit: I noticed I forgot to switch language but you can still see
After the ICS upgrade I get 10% less battery if I use the phone a lot or 20-30% less if I don't use it as much.