The company I work for experienced a two hour outage several days ago, with a Rackspace hosted stack. It was a hardware failure of some sort, most likely a disk in our DB server. Nothing is really 100% fail-proof, and each "9" that you want costs an order of magnitude more than the last. I've been happy with Heroku overall, even though their major outage affected me pretty badly last year. They have learned from it, and don't expect they'll get caught by the same thing again. And they are much better at managing servers than I am. ("Dammit, Jim, I'm a coder not a sysop!")