"The CDN itself being slowly-down is vanishingly rare."
Based on...?
It happened to me once a few months ago. It was down for hours. The negative impact was very real and painful and in my opinion outweighed the other advantages of hosting using Google's CDN.
Pingdom tested Google, Microsoft, and Edgecast's jQuery CDNs every minute for a couple weeks and found all of them averaged between 100-150ms to download jQuery[1]. Google's was actually the slowest of those, averaging a turtle's pace of ~130ms from all of Pingdom's datacenters. They're all so close that the Google CDN's overwhelming caching advantage should be preferable though.
More anecdotally, I've been running a few Pingdom type tests myself for a longer period, using uptime tools on few of my servers and mon.itor.us. Except for that brief outage the morning of May 14, 2009[2], I haven't monitored a net-wide outage or even a 250+ ms slowdown.
I'd be genuinely interested in any concrete data to the contrary.
Based on...?
It happened to me once a few months ago. It was down for hours. The negative impact was very real and painful and in my opinion outweighed the other advantages of hosting using Google's CDN.