Hmm, as far as old phones I'd say most Kyocera's are giants pieces of poo.
Newer phones, I haven't much troubel really..just small things that bug me.
I really don't like Windows Mobile in general, so I wasn't a fan of my Shadow.
The OS on my Motorola Rokr E8 was super laggy. I stuck with it for half a year thought because the jog wheel was awesome and the disappearing keypad was fun to show off and mess with.