Also, not all PaaS players will commoditize over the same feature-set. Example in point, Heroku's Java support doesn't allow you to put a .war file up like you can do in Amazon's Elastic Beanstalk.
I've tried a few others. Both Dotcloud and Duostack (before the acquisition). But I always ran into one or two little sticking points or things it couldn't do or things it could do but not easily.
I keep coming back to Heroku, for both Ruby and now Clojure. (And now that their Node.js hosting isn't awful, I'd probably use it for that too if I did something in Node). It makes it so easy for me to get an app out, even with little wrinkles like needing some configuration variables that I don't want to check in to my codebase.