I have a 2 way PS2 - USB adapter.
Both Ps2 joypad can be used under MAME on MOS :D

I have only sometimes strange problems of latency ... I cannot say if it's a problem of MAme or the connector or the USB stack.

I also used my PC gamepad:

http://www.trust.com/products/productpics.htm?artnr=13016

It run wonderfull on MOS! (the analog joystick too)