your Gmail, Contacts, Calendar etc. will still sync with it off. however - other apps may not. the increase in battery life has been debated here with many getting mixed results.
my advice is just to let it be and just enjoy your phone.
I experimented and turned background data off for awhile and did not notice any difference in battery life. Everything seemed to work fine except Slacker would stop playing.
I can surely say that turning my 3g off, my battery life only goes down by 8% in 3 hrs. I only turn it on when I'm on lunch or on my way home. Battery life has improved by a huge % with 3g off.