I heard users says that SGS2 is quite hot while running heavy apps and in this mode battery lasts approx. 5 hours, but SGS2 has a first place in performance. However during normal use it lives long in comparison with other similar smart phones.
Let me add. Yes Galaxy 2 is too big, so most likely I would like to buy iPhone 4s too. Or I'm wait a some ~Galaxy 3 with less screen. I like Android a little bit more.