Very important. I spend a lot of time at my computer and my desktop environment is like my home. I want it to look in a way that I find aesthetically pleasing and it mustn't try to force me to change the way I work because some UX designer decided that their way was much better than everybody else's. Perhaps you can guess where this is going :D but I've tried to like Gnome 3 since it was first announced. I've given it multiple chances but it just doesn't work for me. It feels like they're going down the same road as all "modern" UIs, where only the most basic features are visible and everything else is either dumped into the "advanced" category or removed entirely. On the other hand, I have a coworker who only uses his PC like a tool, and he thinks Gnome is the best DE ever and can't understand why anyone would want something else.

Currently I use KDE and I'm pretty happy with it. It's highly configurable, and I've made it look and feel the way I want. I used mainly Xfce for a long time but now I prefer KDE.