The biggest problem is having to lift both fingers off the mouse to use the right click. It's extremely uncomfortable after a few hours.
Plus, moving from a large, weight-adjustable mouse like the G5, which has 7 programmable control buttons, to the Magic Mouse feels like a huge step backward.
But they're great for short hops with a laptop.
I rest my hands on the desk or pad around the mouse. My three middle fingers are raised, but my wrist, thumb, and pinky (actually Ring Man too, mostly, LOL) are comfy on the desk pad. I only use the middle fingers, and usually only the pointer, to do most of the tricks and clicks on the mouse. Even with my huge screen, I maximized the speed of the tracking so that without moving my hand or wrist, I can scroll over my whole screen without moving my hand. So it works really well for me.
Check out magic preps to get the most out of your Magic Mouse. http://magicprefs.com/ I don’t have everything, just what I like and need.