IMO, i think all flash users who love flash should learn actionscript. I think it's the best and most versatile component of Macromedia Flash MX. Sure, tweening and all that stuff you do with the timeline and objects are cool, but once you know just even the most basic actionscript functions (like buttons for instance,) you can do all sorts of cool stuff (like even making a website or a links menu or even games.)
It's really not that hard, even if you don't know javascript or any other related programming language. I didn't know jack shiz about any computer language. Just start with simple things like using/storing variables and modifying the x and y coordinates of objects. And as kaboom already mentioned,
http://www.actionscript.org is the place to go.
But yea, I think it's good if you start with just the basics of Flash MX, such as tweening, symbol transformations, etc. Once you get a good grip on that, see if you can brace yourself for the world of actionscript