First just let me say that I love the Lumia 900 but I am having some issues with hands-free devices.
I have the S9-HD headset, and no problems pairing or using it to listen to music with the Lumia 900. Haven't tried using it for a phone call, though.
I have the Nokia earphones that came in the package with my Lumia 900. Issue is that when someone calls or when I am making a call, people cannot hear me when the earphones are attached. When I remove earphones during the call, in order to let people hear me, my voice turns robotic and I have to hang up the call and re-dial again without the earphones.
I am having exactly the same problem as Jennai. I think the problem is not with the headset but the phone itself. The headset works fine with HTC One-X. When I used the HTC One-X headset with the Lumia 900, it is the same problem.