Well, I like that it's fast
I personally don't use it because I'm waiting for it to mature just a bit. There's one feature that I absolutely need
in a WM, and XFCE just doesn't have it. That's the ability to maximize a window horizontally or vertically, but not necessaryily both at the same time. XFCE makes you either maximize the window to the entire screen, or waste your time resizing it. That drives me nuts. Aside from that it's a really great WM
It has a lot of features, too. Very configurable menu bar that you can move anywhere on the screen. It has it's own file browser and text editor which are very nice if you like those sorts of GUI tools, but I prefer the console myself.
It also has nice default themes, so you don't have to spend hours of tweaking to get something decent looking, like with some WMs (*cough*afterstep*cough*).
Oh, and my favorite thing about XFCE is that it's the only WM I've seen that lets you change the colors of the mouse cursor. If there's one habit that I just can't break from my 7 years of windows usage, it's enjoying the white mouse cursor with the black outline. I've just never gotten used to linux's black cursor with the white outline. XFCE lets me make it any color, but I particularly like white