The Roccat Kave XTD digital (or analogue) headset is a true 5.1 headset - or at least pretty darn close. It uses 3 drivers in each ear cup to simulate position, but that's 6 drivers in total, and they work very well (according to me). It works well.
I've never listened using synthetic surround sound using a normal 1 driver/per earcup. The Roccat Kave XTD the headset to beat for me, and I have come across people complaining that synthetic surround is not as good (see Tom's Hardware).
My use case for the Kave headset has often been Team Fortress 2 (an oldie but a goodie); I have been able to hear the exact direction spies were decloaking from, and turn to meet them (to about 5 degrees of accuracy). It hasn't saved me from quieter spies, but no headset can do that. It's also helped me just get a feel for virtual battlefields that I wasn't getting in stereo.
Now if you want sound quality, then get your sennheiser headphones. But if you want good direction, I recommend a Kave XTD headset. Jay liked them too.
P.S. I know this is an old thread, but no one had suggested a true surround sound headset, and I feel I had something to contribute for any later readers.