The Apple Magic Mouse
My iMac also came with a Magic Mouse. It's flat, sleek, and has no
visible right or left keys. But no worries - Apple didn't go back to
the outdated one-button mouse; it automatically detects which button
you pressed by sensing where your fingers touch the surface. It works
surprisingly well, I found it easier to learn to lift my index finger
when I wanted to right-click than I feared.
But the real magic is the rounded surface of the mouse. It's a multitouch trackpad. You can run your finger up and down the surface to do what the track wheel does on other mice, but it can do more than that: sideways swipes scroll sideways, which makes it very easy and natural to scroll in all directions, for example in Finder windows. Some traditional mice allow rocking the track wheel sideways, but that's not anywhere as convenient. It supports "momentum"; give your window a kick with a quick swipe and it will keep running for a little while. Brilliant. And there are also multifinger gestures; for example a sideways swipe with two fingers means "forward" and "back", which works well in a browser. (Not just Safari, Firefox knows about this too.)
But the trackpad is also a problem because it's easy to activate the trackpad accidentally while reaching for the mouse. I often accidentally scroll a little when I reach for the mouse. This is no big deal in a Finder window, but something like Google Maps or Google Earth will start zooming frantically, and you'll probably drop out of Google Street View and have to set your position and orientation all over again. I wish the trackpad was active only when my thumb and ring finger touch the sides of the mouse. Also, it's very distracting that the drag action sometimes starts half a second late, so you start rubbing the touchpad with no visible effect until it ''catches'' and your screen goes berserk.
I found it very difficult to drag with the mouse. The problem is that you can only hold it at the thin edge where the sides meet the upper shell, which is rigid and forms the mouse's single physical button. So if you need to lift it to reposition it for a large-distance drag - and this is necessary more often than you might think because the optical sensor is high up at the top of the mouse - you are likely to lose the drag because you accidentally release the button. It's very difficult to hold the mouse by the thin edge and lift it while keeping the button pressed; also because the gap between the body and the clicked top is right where you must lift the mouse.
I also found the mouse to be too low and not very ergonomic. Form always wins over function in Cupertino. You can't rest your hand or fingers anywhere on this mouse except the thin and inward-curving center section of the base.
The mouse uses regular alkaline AA batteries. Battery lifetime is not very good, perhaps two months of light use. At first I liked that the mouse doesn't come with a recharging dock but now I wish it did, rather than run on disposable batteries. It's a miracle though how Apple managed to fit two AA batteries into such a tiny package.
When the Mac is sleeping, moving the mouse will not wake it up. Some people may not like that, but I love it. I can push things around on my desk without accidentally waking the Mac. And another good thing is that the Mac sees the mouse instantly when waking up; my wireless acrylic Apple keyboard always lost the connection and took several seconds to reconnect, and sometimes I had to toggle Bluetooth off and on. Bluetooth has a reputation for delays like this, but somehow Apple eliminated that problem completely. The Magic Mouse just works. (Unless you switch it off, then it takes a very long time to reconnect.)
I also run Linux on my iMac, and Linux (like most professional 3D graphics software such as Autodesk Maya) works better with a third mouse button. The Magic Mouse doesn't have one, but www.openmediaboston.org offers the BetterTouchTool extension lets you configure all sorts of multifinger gestures, including the middle mouse click. Very useful! If you have a Magic Mouse, you need this.
Update: ok, I have had it with the flakey touchpad on the Apple Magic Mouse. I returned to my old Logitech. Finally I can scroll where I want to scroll, and it's so nice to be able to reliably drag-and-drop things again. Logitech engineers know how to precision-engineer a good mouse that actually works, not a beautiful objet d'art that kind of does what you want most of the time if you work around the design flaws. In the end it only matters what works and what doesn't.
(My iMac also came with a new keyboard.)