But looking back at the results over a period of years, some hires never reached the level of quality/productivity they had shown in the interview homework. I don't know why this is, one possibility is that some interviewees "cheat" by getting someone else to do the project for them. I don't know if that happened to us, but whatever did happen, the net result is that the interview homework approach is not ultimately all that great.
Now we do something different. As part of the interview process, the interviewee sets and codes with a couple of our developers for a couple of hours. This has its own downsides (some otherwise excellent people have trouble with the stress of being "in the hot seat"), but so far we're much happier with it.
Also, spend a few hours trying to write code with someone, and you have a pretty good sense of the oh-so-desired "cultural fit" without having to ever actually look for such a vague thing.