The main advantages to adding jQuery to your toolbelt would be:
- browser compatibility – doing something like .attr() is much easier than the native alternatives, and won’t break across browsers.
- simplification of usually complicated operations – if you’d like to see a well written cross browser compatible version of an XHR method, take a look at the source for $.ajax – for this method alone it’s almost worth the overhead of jQ.
- DOM selection – simple things like binding events & selecting DOM elements can be complicated and differ per-browser. Without a lot of knowledge, they can also be easily written poorly and slow down your page.
- code quality – jQuery has a huge community and a low learning curve. This is a perfect storm for lots of poorly written open source plugins.
- inefficiency – jQuery is easy to write inefficiently. For instance, using jQuery’s each instead of for loops is unnecessary and could have a performance impact in some cases. Lots of good info about this stuff at JSPerf
- bloat – jQuery is a huge library. Much of the time, you’ll use a small subset of it’s features, and grab the whole library. There are some great alternatives that will give you subsets of the features, like zepto.js, and underscore.js – depending on your situation, you can save some bytes by choosing the right library for your needs.
- Ben Alman’s blog – lots of good best practices here. I don’t agree with all of the, but I learn new things from his blog all the time.
- Bocoup – these are training classes. If you’re near one, go to it. Many of the best JS speakers and teachers teach these.
- Paul Irish’s blog – not strictly JS, but plenty of best practices are written about here. Him and Ben’s twitter feeds are both great to follow.
I’m sure there’s lots more great resources I’m not thinking of or don’t know about, other answerers should feel free to add to that list.