Using 3G phone as modem

Recently got vodafone 3G phone and see it can be used as modem to acces broadband via computer
the Cost is somethink like €12 per 15MB, and €2 per MB after that I think .what kinda of usage would this be?
Has anybody used it??

Was going to get Vodafone wireless but wondering now which would be better?

if you do any type of downloading then get one of the cards. 12mb would be about 3 mp3 songs for example.