As it happens, I wondered if I could get to work with jQuery in a previous post. Looks like I turned lucky, and I was contacted recently to work on a javascript-heavy project. Initially we started working with jQuery, but due to the nature of the project, where in jQuery needs to live together with several other js frameworks, and Prototype being one of them, we quickly ran into conflicts.
There are several tricks to make sure jQuery doesn't conflict with other frameworks, but none of them actually worked satisfactorily. We ended up using YUI instead because one of its' strong points is a non-conflicting namespace. Another stroke of luck - I have been meaning to work with YUI too, but finally got the chance. A pretty tight deadline meant I had to get up to snuff with a lot of JS concepts that I understood only hazily, and it was great fun working with JS again.
JS is a very powerful language, and it's very true that it's a language that wears the syntax of Java, but it's actually a kind of Lisp. It is very much like functional languages in its' design, where functions are first class objects, of course with closures etc. Plus, YUI makes it even more fun - it has a complete set of controls which help you build very rich UIs. Also, if you're interested in JS at all, I recommend reading YUI Blog - a veritable gold mine of JS-related info. A very enlightening blog.