Hi Felix- I'm using a USB keyboard with Garage Band, and also with Logic Express 7 and Reason. My G3 is pretty maxed out, though, with a G4 cpu, RAM and a fast hard drive controller, so maybe that's why it works.
It is maybe a little slow, and you have to "lock" most tracks down (i.e., render all the efx) for simultaneous multitrack playback. But it's pretty OK. No complaints.