Careful. Microsoft (despite their promises, as usual) never actually released a PC driver for the 360 chatpad. This means that you'll have to go through very roundabout ways (IE: homebrew drivers, and the only one i found so far only supports wired controllers) to make it work.
I actually know a few people that bought a PS3 chatpad to use with their 360 controllers (requires a bit of working and bolting to attach it confortably :D), since the PS3 one is a simple bluetooth keyboard that doesn't require any driver as long as your PC has bluetooth or a bluetooth dongle. The madcatz one is good and cheap. I've been considering trying it for a while now.
