I used two of them, purchased at different times, that exhibited serious problems when scrolling occasionally. They might have updated the mouse since my friend and I bought them, though, or it could even be an issue from a certain manufacturing plant.
Other than that, I love it. I haven't seen a mouse that tracks as well as it does. It's also insanely useful for Photoshop, et al, due to its insane precision (2000 dpi liek wut). For as much as I use the scroll wheel, though, that design flaw (or whatever the hell it is) is enough to make me not use the mouse.