Yes of course it is, lol. I wrote an article here for Think Vitamin that you'll find of use: http://thinkvitamin.com/code/tips-on-learning-javascript/
yes.... jquery gives you a command to code frontend script in minimum time and provide full user manual. I am preferring jquery to use in a projects. e.g if you want to get value of any field by using id in simple javascript you have to write document.getElementById('FieldIDName').value but in JQuery you simple write $('FieldIDName').val() to get value of the field. if you want to learn more about jquery visit jquery [dot] com.
Yes, is VERY usefull. Gives you a lot of power. Of course you can do all the same with just javascript, but you need to code from scratch and also manage everything to make it cross browser compatible. Well, JQuery already is compatible, powerfull and easy (at least its friendly).
Its a YES for me, theres a lot of cool stuff that you can play around, better and adequate documentation, support and updates from the developers and contributors and even big events. Start to learn jquery NOW!!!
jQuery may be useful, it depends what you need to do. jQuery may slow down the speed when you compare it with pure javascript. Also it needs to be loaded (30 Kb is like a small image ). But it solves browsers javascript incompability and it has a lot of great prebuilt functions. I work daily with jQuery and I like it. In Web 2.0 apps jQuery helps a lot!
whether you are a web designer/developer/programmer, Jquery saves a lot of time, gives your site/application cool interface and it's easy too.