Magic Mouse has been my favorite mouse ever - needed to learn to hold it with finger tips in the middle instead of cupping it. Subtle difference but made all the difference. The inertial scroll & swipe for back/fwd in browsers are killer. It still takes me a bit to remember I can't do that at work on my Logitech/Win setup.
That said, my home desktop is un-ergonomically tall, so was eager to try the Magic Trackpad. Got it. Frankly was kind of disappointed because it seemed inaccurate and took a lot of effort. I was using it like the trackpad on my MacBookPro - using pointer finger for the cursor, which meant making an effort to keep my other fingers from touching. Then hit on using my middle finger (the FU finger) for cursor movement, and have to say it's like controlling the cursor with my brain with effortless access to scrolling and the other gesture controls. Accuracy = mouse this way. And tho some review or another said large screens or multiple monitors are a problem, that's bogus - my center monitor is a Dell 24" flanked by 2 20" monitors left & right (yep 3 monitors) and with tracking set to high it's easier to get everywhere with the trackpad than with the mouse.
Being left-handed mouse options are limited tho Apple has always looked after us. (Until, ahem, iPhone 4 but Apple Mobile platform is another story - not a fanboy and not a fanboy of that platform)
So just getting it out there, it's a great device, but not an exact analog to the MBP trackpad. Experiment. And try using your middle-finger, as an added bonus using your middle finger on occasion may help keep the formerly "beleaguered" Apple humble.
iMac CD/MacBook C2D/MacPro8x2.8/Mini Mid-2009, Mac OS X (10.6.2)