Over at 360Insider they have put up their full review of the Plantronics GameCom X30 Headset for the Xbox 360.
In my testing both the sound and mic quality are superb. The Halo 2 headset had an in-ear design which provided great voice audio quality, but muffled the sound from your TV speakers for whichever ear the headset was in. The X30 headset provides similar audio quality, but since it doesn’t go as far inside the ear you can still hear all the other crucial game sounds. Regarding the mic, the noise-cancelation seemed to work well and others in the game could hear me loud and clear.
In the end they liked it because of its great audio and mic quality, as well as how light and comfortable it was. The thing they mentioned disliking was that the volume control was in-line rather than at the plug.