App Engine has a lot of really awesome features going for it. There are some pains for sure, but not the ones you find people typically griping about. The datastore and all of that aren't a big deal. What's annoying is the lack of library compatibility, and no real SSL suport. Having access to a relatively small subset of available libraries for Java/Python sucks, each time you find a library you'll have to find out for yourself it it works on AE or not.

As for SSL. I mean come on. It's 2011, it's absolutely unbelievable they don't support SSL for custom domains. Unbelievable.