My situtation is a bit unique, because I'm using EC2's for Windows server hosting.
I tried using something like Web Faction or Slice host for a while. And, while they were fine, my *nix admin fu < Windows admin fu. So, it was taking me five times longer to set up a Unix server on slicehost that did what I wanted it to.
I was frustrated, so I tried to set up a Windows server on EC2 running a WAMP stack. I had my site up and running within a few hours. I looked around for other Windows hosting solutions, and about my only other options were to get a dedicated box at 2x the price.
It's been running flawlessly for a month. I'm really happy. I really don't see my self using anyone else for hosting right now. If Azure starts looking ready for prime time, I'll check it out, but right now, it's EC2 for me.
So, not only is EC2 more convenient for me, it's actually cheaper than other Windows hosting options that I found.