I own a 2007 Macbook Pro, and although I also have a mouse, I have gotten used to the trackpad. I could not picture myself without a trackpad again, so I bought the Magic Trackpad and I'm loving the way it works.
Of course the Magic Trackpad makes my laptop's trackpad feel like dredging through a swamp now in comparison.
The real benefit of the trackpad is the gestures, and although the Magic Mouse can do gestures as well, it's not very securely positioned and much smaller for 4-5 finger gestures or sideways swipes and whatnot. I also like to use 2-finger swipes to move around in a big picture (when the scrollbars are present both horizontally and vertically) such as in Photoshop. It is also nice you don't have to move the Trackpad like you do with a mouse, it just keeps stationary. However, I suppose it is a bit more annoying for precise clicks. Actually, I'm not so sure. Perhaps I just suck with precise clicks at times. Anywayyy, I'm loving the Trackpad, and soon I will also be testing the Magic Mouse with my new iMac, so I suppose I can provide a better comparison.