While interesting (and different results than I expected), I would say uptime here is even more important than access times: I'd rather have all my visitors wait 0.04 sec more, than a small percentage not having the page work at all.
Another issue off course is that the files will be cached if your users have visited a site that uses the same CDN. I would guess that the Google CDN is most widespread, and a local cache hit is always faster than a CDN.
Curious - considering that these companies pay for this content and bandwidth - are they actually offerring it up for public use, or is this just leeching off them?
The speed benefit is probably non-existant for something small like jquery - some simple far-future expires headers and proper cache management and site engineering will take care of that.
Further - including scripts from foreign sources in your pages also means you are trusting them to be secure - if that script is compromised, every site using it could be hijacked.
