I'm wondering about the "Easy" button. How frequently do you guys use it? I find myself almost never using it because I'm not sure what it does (it's missing from the documentation), but I'm starting to think I should because I have quite a few kanji to review every day.

I use it when I come across something like 私はボブだ or これボブのだ. So, basically, I mark easy for the things that are truly easy.

I think it extends the time before that particular kanji become expired, so when I see a kanji like 一 I always click easy because its easy.

You should only use Easy on kanji which are REALLY easy. The button does indeed extend the time before the kanji is shown so it should be used to light the weight of your reviews. Of course, you should only do this when you know a kanji so well that you don't think it will be a problem.

I never use it on this site since I have pretty few reviews every day, extending the review time of a few kanji won't make a big difference.

IIRC it's 1.5x the usual timeout time for the next level. I use it for everything that is, well, easy.

~J