I tried out a Magic Trackpad and wasn't overly impressed. I have the full wired Apple keyboard with the number pad anyway so I don't have room to use one in addition to my Magic Mouse. I need the precision of the mouse.
Despite all the negativity I read about it, the Magic Mouse has been great for me. Even with my large hands I don't find it the least bit uncomfortable. I wonder if some of those complaining are trying to grip the mouse, which would defeat the multi-touch function anyway. I grasp mine between my thumb and ring finger when moving it, with my index and middle fingers hovering over the surface for gestures. The heel of my palm rests on the edge of the keyboard tray. With BetterTouchTool I have about 12 different gestures assigned to finger swipes, clicks and taps.
It works perfectly for me and now I find it very difficult to work on other computers without a Magic Mouse, constantly trying to subconsciously do finger swipes.
I have a very ergonomical Logitech MX Revolution mouse that I used prior to the Magic Mouse and haven't missed it. I gave it to the wife to use with her PC.