Trackpad + Mighty Mouse = Win (for me)
I got the Magic Mouse with my iMac purchase 2 years ago, used it for 1 week then put it in the drawer. The design was beautiful and the touch functions sounded promising, but the ergonomics of it blew. I missed the pinch and roller ball click functions of my MightyMouse. And the sharp edges and slim profile contributed to a lot of hand pain with extended use.
So when I bought my new iMac last month, I decided to select the Trackpad option. I love it for scrolling around and the gestures are fantastic. I do a lot of web and Photoshop work and it has been a great addition to my desk. BUT...
For detailed work and text editing, I find the lag and lack precise control frustrating. It seems to require a very light touch as not to click on things randomly. This forces a tension in your hand and wrist to hover over the Trackpad. A mouse will take the weight of your resting hand without errant clicks or movements, or share that weight with your desk. Not so with the Trackpad.
The other downfall of the Track pad - functions that I heavily rely on with my Mighty Mouse's 4 buttons can't be programmed into the TP.
So for now, the Trackpad is only part of a team of input devices, not the star.
For me that team is a Apple Wired Full Keyboard, Wired Mighty Mouse and Trackpad, and occasionally a Contour Shuttle Pro when I need to automate over 15 keystroke sets for production scenarios. I choose the wired versions because I use the Numeric keypad all the time and I hate changing batteries, and the Wireless Mighty mouse felt too heavy for extended use and contributed to hand fatigue.