I've seen Chris mention on Twitter that Google can be a little slow at times, so I'd recommend hosting your own scripts, it also means making requests to less servers which might also speed up the page loading. :)
I've expirienced quite a long delay while inluding a SWFObject library from Google, so had to move it to own server. So, my opinion is that it is better to host libraries at your own.
But, hosting at Google has some advantages too. So consider pro and contras in your own case.
My opinion on the matter is unless your server has an extremely slow connection or you have limited storage/transfer, there is no need to host your scripts elsewhere.
Google hosted javascript libraries
or
on our own server hosted
But, hosting at Google has some advantages too. So consider pro and contras in your own case.