There are loads of rants out there that are easy to find, but personally it's mostly: you can't use it without looking at it to make sure the buttons are what you think they are (nearly always context-sensitive, often surprising when it decides it's a new context), and where you think they are (can't go by feel, so you need to visually re-calibrate constantly). Button size and positioning varies widely, and nearly all of them have a keyboard shortcut already that doesn't require hand movement or eyes (or at worst previously had an F-key that never moved).

The main exception being things like controlling a progress bar (mouse works fine for me, though it's a neat demo), or changing system brightness/volume with a flick or drag (which is the one thing I find truly better... but I'd happily trade it back for a fn toggle and F keys). But that's so rarely useful.