Summary and Conclusion
For starters, Windows users need not apply. While the pad works just fine on Windows machines (XP, Vista and 7) if you put in some extra legwork, the gesture support is so limited (just scrolling on Web pages, for example) that we would have to recommend a pad that's actually made for Windows machines. There's no compelling reason to purchase this one over any of the Windows-centric alternatives.
But for OS X users who have upgraded to OS X 10.6 already, we can certainly see people taking advantage. For instance, if you have a Mac Mini hooked up as an HTPC, the Magic Trackpad makes for an outstanding controller. You won't have access to a keyboard, but if you have your media arranged in iTunes or another scrollable list, it's easy to select the media you wish to play via this pad. This is undoubtedly our controller of choice for Mac-based HTPC setups.
For heavy MacBook users who travel a lot, yet long for a secondary controller aside from a mouse, the Magic Trackpad also makes a lot of sense. The Magic Trackpad doesn't need a specific surface under it to work, and unlike mice (which require a great deal of surface area to function), this pad requires only the space of the unit itself (around 5.2" x 5.1"). It's also super thin, making it perfect for lightweight travel. If you're a Mac laptop user, and you feel cramped with your existing trackpad setup (or you just own an older MacBook or MacBook Pro without the new "glass trackpad" gesture features), the Magic Trackpad is a fantastic travel companion. We still prefer a mouse, but oftentimes mice are inconvenient to use in unpredictable travel circumstances.
But really, will the Magic Trackpad replace your iMac or Mac Pro mouse? For the vast majority of users, the answer is no. It would take a bit of practice on the pad to become as precise and quick on it compared to how quick most people are on their favorite mouse, and many third-party mice offer lots of extra buttons for easy macro arrangements. If you simply prefer the feel of a trackpad over a mouse, then the Magic Trackpad will work admirably. We had no usability or performance issues at all, and the battery only drained 1-2% per day after long, normal work-day-use. The price also isn't outrageous; it's at the high end of what we would pay for a device like this, but for those in need of something of the sort for travel or HTPCs use, it's a decent value.
It's a novel concept, and it has its places, but we can't recommend it as a replacement for those who love their existing mouse. If Apple would have included a capacitive stylus and allowed this to work as a pen digitizer (much like Wacom tablets), we may be singing a different tune. That would really add to the functionality; but alas, Apple included no hardware or software to turn this into a pen recognition pad. It's an opportunity missed, in our estimation. If you're curious about it, we would recommend stopping by an Apple store and trying one out, but unless you detest your mouse setup, you'll probably still revert to what's old, tried, trusted and reliable.