(Just so I don't get myself into trouble again, I should point out from the start that I have a strong anti-Apple bias. I'll try to remain objective, but you've been warned)
I used the mighty mouse for 2-3 weeks at work before hiding it in a drawer and bringing my own mouse from home.
I found the worst thing about the mighty mouse to be that you have to lift your left finger off the surface of the mouse to do a right click. You might get used to it after prolonged use though - I probably never did because I have a normal mouse at home.
The weird buttons on the side act as a single button, and they don't do anything when the mouse is lifted off the table, which can be a little confusing.
Also, I find the little click noise that is emitted by a speaker inside the mouse (I'm not being funny, there's seriously a little electric noisemaker in the mouse to make click sounds) when you press the buttons to really grate on me - there is no way of turning it off.
The scrollball can't really scroll diagonally, you sort of have to stop scrolling in one direction to scroll in another. Also, you can physically roll the scrollball without enough pressure to actually cause a scroll. I got used to that quickly enough.
Also, most people won't notice this one, but if you use the scrollball in an X windows application, you have a decent chance of locking the entire X server.
(Though this behaviour has not gone away since removing the mighty mouse, it may have come with the mighty mouse drivers because I never noticed it previously)
If you find the mouse is just too slow, it can easily be sped up in the system preferences panel.
If you find the cursor doesn't keep up with your hand during slow movements and overshoots badly on fast movements then what you're seeing it the result of Apple's mouse acceleration curve, which can be disabled by purchasing software like "USB Overdrive" - which didn't work with the mighty mouse last time I tried it, but might now. There's also an application called "steermouse" you can buy that might be able to change the acceleration curve. (The windows XP name for mouse acceleration is "enhance pointer precision").
If you go with a third party mouse, maybe avoid products by Razer, as my mac can't wake from sleep when it has a Razer Diamondback installed on it.