I have to say I have the Magic Mouse and it's incredible. I hate to be the bad one here, but I have to critique the above comments by Speedbird1:
1. "it has a thin cable"
- No it doesn't. It's totally wireless.
2. "does not move easily...must apply lots of pressure to move the cursor"
- How so? You just slide it like any other mouse. Unless you're pressing down really hard while trying to move it...
3. "everything turns blue on your screen and you lose seeing the cursor"
- That's called the Blue Screen of Death. It only happens on Windows. You must be getting them confused.

Seriously though, there's nothing I know of like this on the Mac, and certainly no way to trigger it with a Magic Mouse.
4. "if you touch the wrong part, you lose the window you're working on"
- If you slide one finger along the surface, it acts like a scroll wheel. If you swipe two fingers left or right, it activates a back/forward gesture, useful for web browsing. Besides that, there are no other ways to accidentally trigger anything unless you install something like Better Touch Tool, in which case it's your problem if you set up a gesture you can't work with.
5. "your screen splits into 3, showing the last 3 sites that you visited"
- This type of 3-way split history doesn't exist unless it's a third-party application, and again, you'd have to set the Magic Mouse up specifically to trigger this. The most you can do with sites on the Magic Mouse without custom configuration is go back and forward one page. That's done by swiping left and right with two fingers.
So to summarize: since the Macs at the Apple store are not custom-configured using Better Touch Tool or any other special utility, the five statements by Speedbird1 that I have noted above are completely false. There's one valid statement though: "My fingers hurt after using for a while". This may be. With the Magic Mouse, you either like it or you don't, just like with any other mouse. If you can get used to the shape (as I have) it's a truly enjoyable mouse. If you can't, then there are always alternatives. The Logitech MX Revolution is pretty nice but the scrolling pales in comparison to the Magic Mouse (at least in my opinion).
As for the best position to use it in, my experience is this: place your index and middle fingers on the surface about 1/3 to 1/2 of the way back from the front (the apple logo should be towards you). Your fingers should NOT be laying on the surface - only your fingertips. Your knuckles should be about an inch and a half above the surface of the mouse. Now rest your thumb on the side of the mouse, about halfway to 2/3 of the way back from the front. Now grip the right side of the mouse with your remaining two fingers. Use your thumb, ring finger, and pinky finger to move the mouse around the surface. Once you get used to using your fingers and not your palm to move the mouse around, it begins to feel natural.
So my best advice? Try it out. Chances are you'll get used to it and at least find it acceptable, if not better than other mice. If not, then I can't force you to like it.
I hope some people find this helpful.