Just to clarify. General CDNs tend to be a good idea of you are having latency issues.
Standard libraries for major JavaScript project, all served from a single shared CDN (like Google, maxcdn, cdnjs, etc) also tend to be called "CDNs" but this is a little confusing. Yes, these shared files are often stored on a CDN, but that's not the so-called major benefit of these shared hosts. The main benefit is supposed to be that, if everyone references the same copy of jQuery on one of these shared hosts, the when visitors hit other sites, jQuery will already be in their browser cache.
This rarely works in practice. The URLs to these shared libraries. Multiple shared services. Multiple version numbers. HTTPS vs HTTP. The net result is that the probability that someone visiting your site has a copy of the exactly same resource referenced via the exact same URL is very low.
With the overhead of having to do a DNS lookup, a TCP connection, TCP slow start, its rarely worth it. Just concat/minify into your own block of JS served from your own systems. Shared JS hosts/CDNs are a terrible and annoying hack, all in an attempt to save 50KB or so