I've used jQuery, Mootools and Prototype/Scriptaculous extensively and I have to say jQuery is my favourite. I can't really compare to extjs since I haven't used it.

You're right, the documentation is much much better than the others that I've used. That alone IMO is a reason to use it.

Two more reasons:

• I find that with jQuery you tend to write less code than you do with Mootools or Prototype because of the jQuery object and selectors.
• I find you can do client-side processing more easily. I've been working on is a web app that let's you filter through a large amount of data. I've started to attach things to pieces of data using hidden divs and/or classes, and then with jQuery use selectors to apply filters instead of having to make requests back to the server and get SQL to filter them. Takes a huge load off of your databases. You can do this with the other JS libraries, it's just very elegant with jQuery selectors: $(".user .male").hide() will filter out any male users from the list.

There are reasons not to use it too. It is higher level, so if you're old school and want to do things the old school way, then you'll probably have to fight a bit with jQuery to do it. When I first tried jQuery I hated it because it wouldn't let me do things the way I always had. However after you get used to it, I think it is an investment well made.