For what it's worth, my girlfriend is a real designer, and she uses a Mac. She has refused to use Linux for the last 10 years because it was too ugly and not usable.
KDE 4.2 finally changed her mind and she enjoys using it.
It is a really mature and solid desktop where everything fits together, put together after 10 years of collected experience. For me, it was the first time that I've felt that the Linux desktop surpassed the commercial offerings in terms of polish (I've always preferred the Unix way, but it was objectively rough around the edges in many respects).
You should try using it on a distribution which actually cares -- or installing from source and adapting it to your use. A bunch of people only know Ubuntu, and then they use the occasional KDE app which then doesn't fit in because it wasn't Ubuntified and then draw conclusions. If you use the KDE desktop the way it's meant to be used, it's a fantastic, stable, integrated, modern and slick experience. KDE 4.5 is what a Unix desktop should look like in the 21st century.
Every time I use a default Ubuntu GNOME desktop, I feel like I'm travelling 5 years back in time.