If you can get some feedback happening you are going to be a lot happier with your handling, I think. We use shaft encoders to ensure the wheels are all turning at the correct speed to give us the motion we want, while I know of other teams who have reported success using the gyro to stabilize their heading.
If you've got the banebots encoders and divider cards on board, we've developed some code that might be able to help, which we are willing to share. Drop me a PM. If not, it is probably easier to just tack on a gyro... perhaps there is someone willing to share their techniques for gyro stabilization of a mecanum drive.
To see some clips of how we move try
http://www.youtube.com/watch?v=gyn9e0cT3-A
Jason
P.S. Some posts surrounding mecanum drives insist that you need a suspension to have a working mecanum drive system. While that may be true on a rough surface, we are quite satisfied with just having a bit of flex in the frame.