Yes, my isFinished() method is implemented to return true if elapsed >= duration.  When I was using yours though, it didn't have a method like that so I had to check against the actual value, which revealed the aforementioned bug.  You do have that onComplete option, but that's very unwieldy to use in a lot of situations, hence my redesign.

And yes, the easing functions are what's doing most of the work, haha.  I actually found more of those in the jQuery Easing Plugin project and incorporated them into my version: Bounce, Elastic and Back (that last one is neat--it purposely overshoots, then backs up).

I also think of timelines, but will not do anything in this direction until I really need it. That would also require some refactoring in the tween.js I think.

Just thought you might like to know Metallix, the latest versions of Scenario have a tween command built in. So if you needed to do timelines for animation, you can just use that. I'm already milking it for all it's worth in Spectacles.

Very cool! A real project is actually the best way to test the tweens Milk it! ^^