There's a bit of mis-information in this thread, and while I don't want to join in the debate, I'd like to set the record straight for any of the newer teams that might be using this thread for information.
Mecanum Wheels can be purchased from two different FRC Suppliers.
- AndyMark
http://www.andymark.com/Mecanum-s/53.htm
AndyMark's Mecanums were first released for the 2007 Season and have been evolving since then. They're offered in FRC relevant sizes of 6" and 8" with a Standard Duty and Heavy Duty version of each. The Pricing Varies from ~$250 for a set of 6" Standard Duty to in excess of $400 for a set of 8" Standard Duty.
- VexPro
http://www.vexrobotics.com/vexpro/wh...um-wheels.html
VexPro's Mecanum Wheels are relatively new compared to AM's Mecanum Wheels, but have proven themselves to hold up reasonably well in the FRC Environment. They're a bit Cheaper than AM's Mecanum Wheels and rely on a primarily plastic construction in place of Metal.
As far as Mecanum's implementation in FRC, they're a decent way to make your drivetrain more maneuverable, as well as adding omni directional functionality within a traditional frame. The increased agility awarded by Mecanum wheels should allow you to move around traditional defense to the point where 'pushing' isn't necessarily required.
All of that being said, when comparing two similar drives, with the only difference being that one Uses Mecanum wheels in place of a traditional wheel, the Mecanum Drive will almost always have less traction than the traditional drive train. This is due in part to a couple of factors, the wheels always being in a constant state of slip (in that they're applying force in a non-fore/aft direction) being one of them, and the other being that each wheel's motor is independent. This means that if one of your wheels leaves the floor, or starts spinning, you're not only putting down the power of (3) motors.