I got a Magic Mouse to use with my Mac Mini a few weeks ago. This is a Bluetooth multi-touch mouse, so it's a touch-sensitive mouse but not a trackpad. This is my first experience with multi-touch (beyond having used iPhones or iPads for perhaps five minutes total).
The mouse is physically shallow; while part of my hand rests on a normal mouse (leading me to be finicky about the size and shape of such mice), if you did that with a multi-touch mouse you'd get all sorts of unwanted behavior. While on a regular mouse the only relevant interfaces are the buttons (and scroll wheel if present), with this mouse the entire surface is responsive to taps and sweeps. Lay your hand across this and you might find yourself inadvertantly scrolling, perhaps at high speed.
At first, and not having thought through all that, the low profile seemed like it would be a problem. However, it is low enough, and otherwise sized appropriately enough for my hand, that to move it I just put my thumb on one side and last two fingers on the other and go. I wouldn't have expected this, but it feels pretty natural.
Even with the touch interface it does support conventional left- and right-clicks (with actual motion and a clicking sound). It does not have buttons; you just press on one side or the other near the end (where buttons would be). Occasionally my attempts to right-click have mis-fired as left-clicks, but that's been decreasing over time so I guess I've learned to get past whatever problem I was having.
The main use of the multi-touch interface (out of the box) is scrolling. I can just sweep a finger along the mouse to zoom whatever web page or document I have open at the time. In fact, I don't have to keep my finger on the mouse; I can touch, give a quick swipe, and lift my finger and the zoom will go an appropriate amount (past when I lifted my finger) and slow down to a stop about when I expected it to. Somebody put some significant thought into that behavior (velocity modeling seems like it would be hard) and it's pretty cool. The faster you move your finger, the faster the scrolling. No documentation told me this, but before too long I learned that I could stop a scroll in progress by just tapping my finger; otherwise this would be too hard to control and I'd stick with paging. And, unlike a scroll wheel, finger-based scrolling works horizontally. Since web pages with horizontal scroll bars are the bane of my existence (accessibility rant redacted), this is a big win for me. But I bet it'll be useful in fundamentally-horizontal applications (editing audio files comes to mind); haven't tried yet.
One surprise from the scrolling: it applies to whatever window the mouse is currently over, rather than the active application. That's a little weird and I hope there's a preference I can set for that.
There is one big problem with the Magic Mouse: tracking. I was finding that sometimes it would move freely in one direction but not another, or that it would suddenly slow down a lot, and was otherwise unpredictable. I had the tracking speed maxed (unlike with my previous mouse) and there was no problem with the batteries, yet this problem persisted, intermittently. That's unforgivable in a mouse, but the multi-touch was cool so I looked for a solution. Google led me to a two-part solution: first, if behavior is erratic, pick up the mouse and blow across the sensor because of dust or cat hair (!). And second, MagicPrefs is a software add-on that not only speeds up tracking quite nicely but also allows you to program all sorts of other multi-touch gestures (which I have not played with yet). With those two changes I am satisfied with the tracking.
Using this mouse is a different experience from what I was used to, but overall it's been pretty positive.