I have an Aiwa 900Mhz wireless headphone. There's definately some loss of quality when going wireless; I attribute it to the 900Mhz. It's still good for playing games and listening to music while I'm doing something else, but I go back to wired headphones whenever I'm in my chair.
Not that this has anything to do with Sennheiser headphones, but those are 900MHZ Headphones regardless.
--Ed