Well, here in Japan the entire network is 3G - there is no 2/2.5G network to fall back on, it simply doesn't exist. So, yeah - I leave 3G on all the time if I want to be able to use my phone.
My battery life is pretty poor. I ran it down to ZERO last night (it powered off in the middle of a movie) and I charged it overnight, in an attempt to 'reset' the gauge. This morning I unplugged it and then went to a coffee shop to relax and to test it out. I turned off wifi, GPS, and Bluetooth (I enabled BT when I got home, as seen in the screenshot below) ... I got the 20% battery warning after 3:05 of use. Here are my usage details:
Four hours since I unplugged the iPhone from the A/C charger, I spent that time listening to music while surfing (all of that data transfer was this morning), playing games, and talking on the phone (42 minutes, most of which was spent on hold with Apple trying to get a refund for a non-functioning app) ...
Since I will generally use my iPhone for music + internet about three hours a day, and since I don't always have access to A/C power (commuting on the train, etc) I guess I'll have to buy a battery pack of some sort. If only they were sold here in Japan! I actually checked on my way home but the huge electronics store by my house doesn't have anything good.