I'm against it for stuff that is not watched like a hawk. For example Google's CDN version of jQuery is fine but I'm not going to use any other random CDN Bootstrap decided to use because I don't trust it.

When RC1 came out, the CDN version had a random dev build (90% broken) but on the page they linked to it as RC1.

I compared the downloaded version .vs CDN and the files were different.

So be careful, if company hosting CDN is not well known or has a good reputation you might screw your self.