The magic mouse is horrible for comfort, and can't support Apple's great gestures unless you install a third party plugin.
It baffles me that Apple doesn't include the Magic Trackpad as the default mouse/trackpad for their desktops. It's easier on the hands and allows you to do much more with the computer. Gesture are easier, more intuitive, and don't incite pain.
I'm pretty sure it depends on how you 'interface' with the mouse -- the Magic Mouse doesn't feel painful to me at all, but I also don't rest my hand flat against it.
I also like the Magic Trackpad, but navigating a pointer on the screen never feels as natural to me with a trackpad as with a mouse I physically move.
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Finally, someone with a little sense!
Personally, I'll never buy a mouse, or keyboard that doesn't have a nice long wire attached to it. Batteries are just one more thing to worry about/blow money on, simply to save one from "the horrors" of those unsightly cords.
This analysis is spot on for most Macs since the machine is the screen -- iMac or MacBook -- and thus there is a very short distance between the machine and the keyboard and mouse, and wired Apple keyboards all have that nifty feature of having a well-placed USB input for the wired mouse.
However, if you are on a PC box, your big box may be quite a distance from the keyboard and mouse, and I've always found a leashed mouse to be especially problematic in these cases since virtually no PC keyboards (certainly not the stock ones that come with the system) have a USB input for the mouse. I have often found that the box is so far from the mouse that sometimes the "leash" on the mouse (even though it is pretty long) is too short and it literally will affect my ability to navigate with the mouse.
Obviously there are ways around that.... a USB hub comes to mind, and some monitors even have a nice USB hub built-in, but not all do, for example, the ones where I work do not, so I prefer a wireless mouse there.
At my last job, on the other hand, I used my own MacBook Pro hooked up to an external monitor, and I preferred my wireless Apple keyboard and Magic Mouse to wired there because it was one less cable I had to detach at the end of the day.