Overall I've been quite impressed with the Radiant AI - sure it's not perfect, but it does a pretty good job of helping to create the impression of a living, breathing world. The decision to have voiceacting for every character (even minor ones) was a masterstroke - it's little things like that which really add to realism (nevermind that some characters have the same voice or that beggars' voices change when you ask them for rumours).
That said, one funny thing that happened to me was when I was doing the scamp quest in Leyawinn, with the woman stuck in her house. I went and got rid of her object for her, and when I got back into her house (it was about 9pm or something) she first threatened to call the guards, then when I clicked on her she was all thankful (the proper dialogue thread) and gave me a reward, then when I ended the conversation the guards came and arrested me for trespassing!
Same thing happened with the elven statuette quest...