i ditched 3 and went with vodafone when i got my n900 because i wasn't prepared to wait for a fix from nokia.
my total data used over vodafone since november has been 220mb.
and voda give me a better signal and a more reliable data con.

i leave everything online/logged in all the time.
but my heaviest use is at home =wifi, here in the cafe I'm sat in = free wifi.
but all day at work it is on vodafones network since there is no eap-leap support.

so i think i'm going to be ok with the 500mb cap. sure i *could* blow it easily, but my usage means i can do big transfers over wifi fairly eaisly.