I've been using Apple's new 'magic trackpad' for about a week, and I'm liking it more and more. For those that don't know, the trackpad is 'magical' because it's basically a large touch surface that you can control with your fingers. It's like the trackpad on Apple's laptop computers, but bigger and designed for use with a desktop computer.
I have an iMac (in addition to a Mac laptop) so I got the trackpad for that computer. I had been using Apple's 'magic mouse' and liked it, but I craved a larger surface upon which to use touch gestures.
The trackpad does take some getting used to, but not as much for me because I have been using the touchpad on my laptop. I decided to use only the trackpad (i.e. no more 'magic mouse'), so that's another thing takes getting used to.
Ergonomically, the trackpad seems to be easier on my right wrist. Having to grip a mouse, even lightly as I tend to do, does engage additional muscles, which over time get fatigued. If you have a mouse then you have to have a mousepad, which is another benefit. In other words, you avoid these problems: the mouse sometimes runs off the mousepad, or the mouse is not optimally centered on the pad. With a mouse you have to manage the positions of two separate objects on your desk. With the trackpad (assuming you ditch the mouse, and why wouldn't you?) there is only one object to keep track of. And in my book less is always more.
As for the touch gestures, I was familiar with some of them from using my laptop. Two finger scrolling, for example, is much appreciated. But other than that (and steering the cursor with my finger) I had not really delved into all of the available touch gestures. The trackpad accepts gestures based on one finger, two fingers, three fingers, or four. The box in which the magic trackpad is sold has 12 sample gestures described on the back. I kept the box out for reference and found that it reminded me to try those other gestures. I now regularly use 9 of those gestures, and find that I can do things much more quickly.
Can it completely replace the mouse? Yes, it can.
But you will find that in certain programs, where you have gotten used to using the mouse to do some weird little thing, you'll at first feel handicapped. But if you pause and figure out how to use the trackpad to accomplish your goal you'll quickly overcome the pining for your mouse. Strong habits are the hardest to break, and using a mouse is something that most of us have developed deep habits around.
If you're going to use the magic trackpad make sure you have the latest system software on your desktop computer. Apple has pushed out an upgrade that gives more choices on how to configure the trackpad, and you'll want to take advantage of those options. I suggest you enable 3 finger swipe to control the 'click and drag' function. I also encourage you to try the four finger swipes (you'll see why).
The magic trackpad costs $69, which is the same price as Apple's magic mouse. I would definitely opt for the trackpad over the mouse, at least for a desktop computer. I know some people who have magic mice for their laptops, but it seems like overkill to get a trackpad for your laptop when it already comes with one built-in.