I find having both the trackpad and magic mouse to be very useful. For general use I really like the trackpad with gestures, and for more precise work (editing text, games, etc.) I move over to the mouse and still have gestures available. I use them both constantly and interchangeably. I have both devices at every computer in my home.
I also like attaching the trackpad to the wireless keyboard with either the "Clique" from Henge Docks, or the "Magic Wand" from 12South.