YUI3 is much better than YUI2, and they learned a lot from jquery in making it expressive / terse. I even spent the better part of a week learning about YUI3 thinking I might use it instead of jquery. But then I found, and it's hard to explain exactly why, that I was much less productive with YUI3. I think that jquery is just so damn intuitive to use, I usually can guess how something will work. With YUI3 I was often left looking up something in the docs, or looking for an example (which wasn't so bad given YUI's excellent docs, but still, it's a hit when you can't just guess how something is going to work)