Maybe I'm the reverse case, but I really, really like Twitter's UI. Especially their newly redone mobile UI.
While Facebook is now completely unusable on my phone, and Reddit is very close (an example of too-much-JS-gone-wrong), going on Twitter is a blast of minty fresh air at my fingertips.
I'm not a user and I don't think that I have strong feelings about their service but if forced to visit their page I'm confused most of the time.
A bad UI that can't be excused by success ist just a plain, bad UI.