Hate them. Just as Craig demonstrated, you will NEVER, and I mean N-E-V-E-R get the gestures to work on the first try or without triggering something completely different. Even something as "simple" as scrolling is broken.
OK OK. I have left my mouses in drawers for so long and have recently revived them due to changing my work setup temporarily. I'll take this part back. With the latest OS, they respond pretty well. Not always great, due to some sort of desensitizing (like de-bounce for touch interfaces), but way way better than they did when they first came out!
I really dislike that I can't palm the mouse. Holding it feels like cutting a deck of cards...and then cutting and cutting until you're trying to evenly divide a stack of 20-cards with one hand.
What @thomasparra said is still true though. The stair-stepping on diagonal movements is gross.