I have the mugen power 3600 mah battery in my phone. Probably the best investment i've ever made since the 2400 mah battery on my old G1. It lasts me through the ENTIRE day on 3G (I never saw the 4G icon btw. probably because of where i live)
I leave for work at around 8AM, drive to college at 4PM, and drive back home at 8PM. on all trips (30-45 mins), I listen to online MP3 streams on 3G. At work and college, I check twitter, gtalk, G+ messenger, email, and some light browsing.
i hit 15% battery at around 1 AM or so. I never felt so careless about charging my phone :P
EDIT: also, the fat battery cover makes the phone feel far more ergonomic than with the stock battery cover. the size feels just natural for my hands.