Several people have reported it already but with some USB sticks the rt2500usb driver does not work.
Usually debug messages claim "vendor request error", or other hardware related problems messages appear.
A possible solution is to try the rt73 driver. Some of the USB ID's for the rt73 chipset match the ID's for the rt2570 chipset. Hence the reason that the module does detect the device but hits into problems in a later stage.
There is a rt73 GPL driver on the Ralink website and rt73 is also developed in rt2x00. If those don't help either you can post a new topic on this forum so we can see what the problem could than be.