for the moment i have a corded KB but i was thinking about IR for the flexibility. the hardest part is running the wires. My HU has it own IR remote and with a IR keyboard you can program short cut keystrokes into a programable remote and cover both HU control and limited PC control. If you really need to data entry then you can breakout the keyboard.
I have the same keyboard as guitar. It works pretty well in the car, but the track ball feels a bit cheap. Comes in very handy when the touchscreen just wont cut it. I keep mine under the passenger's seat.
I have an ir remote for my desktop. The range/angle makes it a little annoying. I wouldn't recommend it for a car application.
I am setting up my system with all the gizmos, and of course I have a touchscreen (or 2... but the in-dash model isn't available for sale yet) I've tried doing normal things with it, and all you are really limited from is typing. The on screen keyboard with windows is a total pain. If you just want to control your mp3 player or navigation system, you can do great with just a touchscreen.