We haven't had availability problems with our site at Dreamhost, but it's not something that's going to be hit by thousands of people every minute. A few thousand in a day has been no problem though.

Bigger issues come with mail. I've written about it a few times before. SMTP and POP servers going down all the time for at least a month. Now mail connection issues just happen infrequently. But mail, both outgoing and incoming is still being lost. I can't tell how much of it though. The most recent manifestation ended just last week when I noticed that mail generated by our own contact form wasn't always being delivered. Sometimes I'd get the debug log version of the message but not the proper message itself. I have a logging procedure in place to help with fighting spam atttacks. And other times I'd get a BOUNCE back from my own mail server saying that I had reached some SENDER QUOTA. They got back to my support request and claimed there was some misconfiguration on their end and it was now fixed. This took 2 weeks though.

The thought of moving the site right now makes my skin crawl though. Custom PHP installation, a hand full of SQL databases and a large selection of files, most of which are html and php and don't rely on any server-specific paths. Still more than I wan to think about for now.