I've used the library in a real project, and I'm on the mailing list. FYI, there doesn't seem to be much development activity going on, nor is it so perfect and complete that further development is unneeded. That's not to discourage anyone from using it, since it's the only JS lib I know that fills the same niche, just to let you know you may need to get your hands dirty. (I've got a patch waiting for integration but not enough attention available to test it for use cases that are not mine and really push it through. If you do have the interest and bandwidth to polish whatever improvements you make, you could probably help quite a bit.)
I had the strangest bug with this library on Konfabulator a while ago where all Date.toString()'s returned an object instead of a string.<p>Not really a useful thing to know but I wanted to post it anyhow.
That interface... it's so fluent. I love javascript, for just the reason that it's possible to have beautiful interfaces like this.<p><i>Date.today().add(3).days();</i><p>Be even better if it let you leave off the brackets like ruby, but oh well.