I use to turn off 3G whenever I wasn't browsing the net to save on battery. It use to be that I wouldn't quite make it through the day if I used it fairly regularly as a PDA.

Since I updated to 2.1 (pwned) I've found about 20% more battery at the end of my day....which now makes it to the end of the day for me. Not perfect, but I've stopped worrying about carrying around a charge cable just incase my phone freaken died.