First off, thats a holonomic drive (a "kiwi drive" aka a kilo-drive only has 3 wheels as opposed to 4-or any other number-), and it has its advantages and disadvantages vs. a mechanum drive.
The advantage is programming simplicity, in favor of an omni-drive. With a "fixed roller" mecanum system, you have a 50% efficiency loss, as you do with a 4-wheeled holonomic system (unless spinning in a circle, when you are using all motors, therefore have 100%, giving an advantage to an omni-system in a spinning situation). In a system where you can actively turn the angle of the rollers on a mechanum wheel you can obtain a much higher efficiency, but that creates very very great depths of complexity in design, and is nearly impossible with the motor, money, and time limits imposed on FIRST.
Id also highly advice you mount your omni wheels in the corners, possibly even tilted to acheive a much greater support base, reducing the chances that you will be tipped over.
http://www.chiefdelphi.com/forums/sh...ght=drivetrain
see that thread for many details about omni-directional and other systems.