1) On my G2 I didn't notice much of a difference in battery life after rooting, however, I am running a stock rom and I found that CarrierIQ seemed to be burning a lot of my battery. After disabling CarrierIQ, my battery life increase was noticeable. I have not tried a custom rom yet. The stock rom isn't bad once you get rid of the branding clutter and clean up the notifications area.
2) I came from an iphone and it took me about a month to get used to the back buttons. I hated them at first and didn't use them at all, just the knock on feature on the screen. I am used to them now and I actually rather like the location. There certainly isn't anything wrong with side buttons on most phones, but there isn't anything wrong with them being on the back either. I know lots of people complain about the button location. I rarely use the knock on feature now. Only if the phone is setting on the desk and I want to check something without picking it up. You will get used to it and maybe even come to like it better like myself.