I installed Kubuntu 13.10 on a machine at work today, and found the fonts in GTK applications looked pretty awful. I started stuffing around with the font settings, but in the end found that the steps detailed in this askubuntu answer were what fixed it (thanks Rael!).Specifically, this step:
KDE Settings > Application Appearence > Fonts.
Mark Anti-aliasing as "enabled", click on "Configure",
uncheck "Exclude range", check "Use subpixel",
and select "Slight" for "Hinting".
It's strange because I've never had these issues with Kubuntu on other machines, and may have been due to the high-resolution monitor I was using.
Update 9/3/2015: Only just discovered but it seems this change breaks text in Gimp when using KDE. Turning it off fixes Gimp, but it's kind of annoying since the fonts go back to looking poor...