I posted a similar thread earlier. I got the quad-core i5 iMac last Thursday and got a Magic Trackpad to go with it. After trying it in the store, I was hooked!
I've been using only the Trackpad since I got the iMac, but tried out the Magic Mouse last night. I used it for about 30 seconds before I went right back to the Trackpad
That said, dragging and dropping is just better with a mouse, period. The whole tap to drag, tap to release deal with the Trackpad can be a little frustrating when you accidentally start dragging stuff everywhere.
So I plan on using the Magic Trackpad for everyday use, and the Magic Mouse for tasks and apps that require a lot of clicking and dragging that would be better suited for the mouse.
PS - Love the i5 27" iMac

It was huge when I first started using it last Thursday and every day it gets smaller and smaller as I get used to it --- good to know for those who are scared of the 27"'s size. You get used to it.