Happened to be in a major supermarket’s electronics section the other day and was told that the “3” network offers the best mobile broadband.
Although oddly all of the O2 retail hardware boxes were sold out, plenty of “3” boxes to be found.
Unfortunately no reasons were offered by the employee as to this blanket statement.
EDIT:
Except this mobile broadband Group Test at the Register proves him absolutely right:
The clear winner is 3.