I've got the Magic Mouse, and ever since I discovered the Better Touch Tool, with which I was able to customize the gestures, I've loved it. I have considered the Magic Trackpad just because it's so big, and may be better in certain scenarios than the mouse (i.e. uneven surfaces), I'm more than likely going to stick with the MM.
Honestly, before I got the MBA, I was completely adamant about pairing a mouse with a laptop; after using Apple's outstanding trackpad, however, it's really not necessary - more a matter of convenience.