Given Google's reputation of awful customer support, why would anybody want to locked into their platform, especially when you have to do so many things the "app engine" way?
The only thing I would consider using App Engine for at this point would be a school project or something.
If agility matters, I've not found anything that beats it. You might not want your service running on there as you scale, but it's enough to get a few thousand users onboarded as a proof of concept.
If you want a proof of concept and you're not concerned about getting the scalability, why not run on a single server?
[1] https://developers.google.com/appengine/articles/sharding_co...
Well, except that there is an open-source alternative implementation of App Engine that you can run anywhere you want. [1] Which kind of ruins the whole "no way to failover from App Engine to some other platform" argument, even for apps built the "App Engine Way".
To be fair, AppScale is a separate entity and therefore there's no guarantee that they will keep up with GAE API's, API bugs, and/or quirks. These are obviously things out of their control, but I'm saying this because there's still no guarantee of a 1:1 replacement if you need to move away.