I have an extreme. Never got it working with my xbox. It will simply see the extreme but refuses to connect, no matter what settings I try (and i've tried them all - trust me). So I have to use an old express (not the new ones as they wont work for me either) as a wireless extender / station and it connects to the 360 no trouble.
However my 360 wireless dongle is from a launch machine and hence it may be outdated hardware, and newer versions may work. Just not mine.
Other than that it works perfectly on my network with the express.