OP here. I actually used to use App Engine for Soundslice, about a year before Soundslice launched publicly. It was cool in principle, but it's severely limited to the point where I just couldn't run my app on it. Plus there was wildly varying performance with the database layer.

I'm sure it's gotten better since then (this was circa late 2011), but I'd think long and hard about using App Engine for anything nontrivial.