Mostly, I trust the way it's built. Apart from components integrated from upstream, every line is TDD'd and 100% pair programmed.
You can use public installations on Pivotal Web Services (by the company I work for) or BlueMix (by some plucky startup from Armonk). Or you can install your own on OpenStack, vSphere or AWS.
Now granted I pay about $300/month for the VM I have with my hosting provider, but I can pick up a phone at any hour and talk to a tech when things go south.
I really don't care how good a service is, I want to talk with a breathing human being when my business is down.
High-availability, automatic failover, managed, with diamond-cut SSDs I hope? Who are you with? For that money you should just pick up a box.
You do get support for no additional fee, but you can pay for additional access to support staff. That said, they've always been quite responsive to our requests, with a fairly short turnover time by email.
It is however customary in the industry to pay for additional levels of support:
https://aws.amazon.com/premiumsupport/ https://www.heroku.com/critical