I actually re-watched it yesterday because it was raining and it felt like the thing to do. It wasn't as good as I remember, but I agree that the dialogue was one of the stronger points.

Julia Stiles with David Cross was a bit of a stretch, but I think it was partly to outline the fact that she's been with a lot of crazy guys in the past and was trying to settle for a more normal boyfriend. Maybe she reasoned that a "less cute" man at this point in her life would be more likely to be both single and mentally stable.

I've got to say that I enjoyed the ending. [vague spoilers ahead] It kept to what I thought of as the theme of making yourself comfortable in unforeseen and awkward circumstances. That's not the best way to put that, but I think it's clear that none of the characters are where they thought they would be in their relationships, and that the ending was a kind of collective and grudging acceptance of everything going on. Plus, I can't think of any better ways to end it without taking away from the realistic universe that it's set in.

And Shane's paranoia/conspiracy theory attitude definitely made him my least favorite character. They could have fleshed him out a bit more rather than just using him to be an annoying foil (for pretty much everybody's laid back attitude) to shit on.