I've been using EC2 and more recently App Engine. While AE isn't suitable for things like render farms, if you are in a situation where you could use it, I would really recommend trying it out. No longer will you have to even think about how many servers you need, how much space you need or such considerations. You just magically have enough.
With all the discussions about how Google turned off Gmail/AdSense accounts for no reason, I'm afraid to use App Engine. What if some bright mind or bot decides to kill my Google Account? I won't have access to my app anymore.
I like to have Plan B¹. If I were locked out of my Mosso Cloud Server, I would switch to EC2 or any other VPS in a matter of hours. This is not true with App Engine -- where would I go?
Also, App Engine has an Google-specific API. What if they decide to deprecate a part of it? This happened before with their Blogger API (I'm an author of blog client, and they I had to write new code to use it) and search API.
P.S. And sorry for offtopic.
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1. E.g. I use only one Mac, but have 3 of them. This is mostly because they break every three months, and I need a replacement quickly (I'm not crazy, this is how Apple's service works in Russia: 2-6 weeks for a trivial repair).
I appreciate the point he's driving home, but his rate of failure is meaningless without knowing the size and uptime of his "server farm". Is it 10 servers or 100 servers? Have they been up for a month or for a year?
I pull this point out specifically because I personally had 4 EC2 instances epically fail on me over the course of 18 months and 40 instances. Of course, this was in year 2 of EC2, so I'm sure things are better now, but I believe it's an important data point.
