The general impression, as I've understood it so far, is that App Engine has its limitations, but Google makes you well aware of them, so adapt to them and don't get frustrated or surprised by them two months into development, or host your service somewhere else like AWS.
I did a quick !hn search for you[1] and saw these three popular submissions:
http://news.ycombinator.com/item?id=1934563
http://news.ycombinator.com/item?id=1931456
http://news.ycombinator.com/item?id=1927903
The general impression, as I've understood it so far, is that App Engine has its limitations, but Google makes you well aware of them, so adapt to them and don't get frustrated or surprised by them two months into development, or host your service somewhere else like AWS.
[1]: http://searchyc.com/submissions/app+engine?page=1&sort=b...