I have a confession to make. I haven't seriously played a first-person shooter on a game console since GoldenEye on the Nintendo 64. I just couldn't get into the controls. It was awkward enough on the N64, but circle-strafing with dual analog pads just feels wrong to me.
The modding group XCM has unveiled the prototype for the XFPS 360, a PS/2 (the keyboard/mouse blug, not the gaming system) adapter that works with the Xbox 360. It plugs into one of the Xbox 360's USB ports, and offers the PS/2 keyboard and mouse support, as well as two additional USB ports and a PlayStation 2 controller port. Plug in your favorite keyboard and mouse, load up Halo, and start shooting like you are on a PC.
Of course, all new products work well until you try out the real versions. XCM has a very pretty prototype, but if and when it puts out the final product, we'll see if you can actually game old-school with the Master Chief.