http://www.wildstang.org/2002/video/video2002.htm
Above is a link to the WildStang videos page of our 2002 robot. If you watch some of those videos you may be able to get a good idea of how crab drive works. The first video on the page shows the drive pretty close up and might give you an idea of the speed.
As stated in the post previous to mine, the drive will turn as fast as the steering motors allow it to. The speed is also determined by how much friction lies between the wheels and the floor. A lighter robot with little tread will require less power to turn resulting in a possible greater turning speed.
For our robot we used two globe motors to turn the crab modules. One was used to turn the front two modules and one for the rear two.
To control our crab movement we set up two joysticks. One was set up to control our 'regular' drive movement (Forwards, Backwards, Turn Right, Turn Left). The other was used for 'crabbing' or 'strafing' (X-axis controlled modules rotating clockwise or counterclockwise). This made for greater control over robot movement and also easier driving.