I know what you're thinking: I've got a nice new(ish) Apple Magic Mouse, do I really need another pricey Apple gadget? The answer is no you don't, but the lure of anything that comes out of Cupertino is hard to resist, and why should you?
The first thing to note is that the pad must be paired with your Mac (by clicking on the Bluetooth icon in the Menubar and then Set Up Bluetooth Device) before you run Software Update. After doing this, System Preferences will be updated with the Trackpad's preference pane, giving you access to its multi-touch options.
Even if you're accustomed to a MacBook's trackpad, the Magic Trackpad still takes a little getting used to. The MT is larger than a MacBook's pad and has to cover a greater area of screen real estate. While using a trackpad is quite intuitive on a laptop, with a desktop grabbing and clicking on a mouse is so ingrained in you that the MT is a bit disconcerting at first.
It's worth persevering though. Gestures on the MT - two-/three-fingered swipes left/right to navigate between pages, three-/four-fingered swipes left/right to switch between applications, four-fingered swipe up for Mission Control, four-fingered pinch for the Launchpad and the opposite (spread) to show the desktop - are numerically superior to the Magic Mouse and are usually much easier to carry out.
There are two other points about the MT you rarely see in reviews: with Tap to Click enabled, it's silent. After a couple days of using the MT, the Magic Mouse, in comparison, seems to scrape and clunk in a way that's positively primitive. For anyone who appreciates working in a quiet environment, the silence of the MT is immediately noticeable. Secondly, getting a Magic Trackpad doesn't automatically mean retiring your old Magic Mouse. The latter is better suited to some tasks (particularly the selecting of areas and the twiddling of virtual knobs in programs such as Logic Pro), and if you have a desk with a pull-out shelf the two devices can be used in tandem very easily.
So, while the Magic Trackpad may not be strictly necessary, it's still a nice complement to any desktop Mac system.