I can safely say that I am a desktop customisation addict.
The problem is, I'm not entirely sure that it's because I actually enjoy it, or whether it's because I am completely OCD and just perpetually in pursuit of pixel-perfection.
It's fair to say that one of the things I love most about working with Linux (and the open-source community in general) is the extent to which you can customise everything to your liking. And I do mean EVERYTHING. (I think my favourite obscure thing to customise has got to be the spacing between the menu and submenus in Openbox...Awesome!) HOWEVER, if you're a little perfectionistic like yours truly, it can also be the bane of your existence, because after a while, you start to notice every pixel out of place. Or how many times have you downloaded a new GTK theme, only to find that, for instance, the Google search box on your Firefox toolbar ends up being the wrong colour for some unknown reason?
The sad fact of it is that because Linux distributions are largely written and edited by the Linux community, many of whom do their coding in their spare time in front of the telly, there is rarely the attention to detail in GUI design as there is in a commercial OS like Mac OS or Windows. I hate to admit it, but while those desktop environments only give you a few choices of looks, overall, those choices are much cleaner and more complete. Linux distros tend to be easier to customise (in fact, they usually encourage it by installing several default themes), but the overall impact is weaker because they tend to be standalone themes, rather than part of a whole desktop "look". Add to this the awkwardness of GTK vs. Qt, incompatibilities with Firefox, quirks of various panels and window managers and, well, you can get a bit of a mess. So really, desktop customisation is a bit of a double-edged sword: whilst you can tweak your GUI to your heart's content, more often than not, I find that each tweak uncovers something else needing to be tweaked, and it quickly becomes a downward spiral...