I use a Logitech MX Revolution with my right hand, and a Magic Trackpad with my left hand. (Bluetooth keyboard in the middle)
I am a designer. Constant Photoshop, After Effects, Final Cut Pro, etc. I use the mouse for most everything and then I use the trackpad exclusively for gestures. I find it extremely helpful in Photoshop. Whenever I need to zoom in, I don't have to go grab the magnifier and then the hand to move around in the zoomed project. I just use pinch gestures and two finger scrolling on the trackpad!
For me, it's the best of both worlds. I don't get hand cramps because the MX revolution is so comfortable and my other hand doesn't have to hover over the trackpad like it would if it was my main mouse. Instead, it only goes over the trackpad when I need to zoom or scroll.