Well, I broke down and got one.
The XCM Cross Fire adapter lets you connect your PS3™ controller to your Xbox 360™ console. With built in rapid fire / turbo,and tilt functions, this adapter is the ultimate in Xbox 360™ accessories. This adapter is also compatible with the Xbox 360™ and Original Xbox™ wired controllers for added Rapid Fire / Turbo functionality.
I'm looking to save my fingers more than anything. I'm starting to develop pain in my right hand. :hs:
Here's a demo video of what rapidfire does on H3. (Note: kill the sound)
The biggest differences are:
Speed of BR / Carbine clips.
Speed of Chopper shots.
Speed of Ghost shots.