I tried the Magic Trackpad at a store here and it wasn't for me. I found selecting text for copy/paste, etc awkward and it isn't nearly as accurate as a mouse. I've been using the Magic Mouse with BetterTouchTool since I got my 27" iMac in November, 2009 and have about 14 gestures programmed.
I changed them slightly with the upgrade to Lion from Snow Leopard but otherwise it was a smooth transition.
I read a lot of complaints about the Magic Mouse not being ergonomically comfortable but I have very large hands and personally find it to be very comfortable. When not moving the mouse my wrist actually rests behind it on the desk leaving my fingers free for the gestures.
2-finger swipe up for Mission Control, 2-finger swipe down for App Exposé, 3-finger swipe right for Close Window (cmd-W), single tap left and right for volume down and up respectively. 3-finger swipe up for Dashboard, 4-finger swipe up for Launchpad.
Anyway, the point is there are more than enough available gestures for use with the Magic Mouse. I can't imagine going without it now.