How well does the Apple Magic Trackpad perform as a mouse replacement?
I've found the trackpad to be great for general purpose use (particularly the 3-finger swipe-to-drag gesture and the wonderful inertial scrolling) but I've found it too inaccurate for GUI development work. 

When I'm laying out screens for software development, or creating graphics for said software, it's too awkward to exactly position things with. I suspect you will find a similar problem if you are touching-up images etc.

I've found the ergonomics to be good, to be honest - I mostly use the Magic Trackpad and when that's not suitable for a job (see above) I switch to a Magic Mouse for a couple of hours. I have relatively small hands and find the Magic Mouse to be a really comfortable mouse, with no strain on fingers or wrist even after prolonged use. The trackpad gives you less to 'lean on' but I haven't found myself with any wrist ache or strain even when I've been using it solid for a day, etc.