Even though o2 states "unlimited usage" on their tarrifs, if you read the small print, it also states "Fair usage policy applies". Same with every ISP.
As for your last comment, that's really nasty....there are a lot of regular members of this forum who use VM and we're all satisfied. They've never let me down in seven years (and more if you include telephone/tv services).
I've also read bad comments on o2's service, there are bad comments on all ISPs!