[Lil' Lavery]

Consult many other threads in the forums concerning omni-diretional mobility systems such as mecanum, holonomic, and swerve drives.
In short though, Mecanum drives use wheels with angled rollers to divert some of the wheels velocity sideways. Because of this (a 45deg mecanum wheel) they are roughly about 70% as efficient as a scrub steering system with identical geartrains.
If all 4 wheels remain in contact with the ground, they are usually reliable. Many teams (such as 40) have created active suspensions to keep their wheels in contact with the ground.
Mecanum drives were able to climb the ramp on a number of bots in 2006, such as 357 and 40.
Bots with mecanum drives had varying levels of success, but generally did fairly well (as mecanum drives are also usually built by veteran teams with a great deal of expierience anyway).
Check out what teams 40, 357, 488, and 868 did.