Honestly: I like that it means I don't have to muck around with configurations, the fact that downtime is out of my control is frustrating, but at the same time, if I were running things, there would be more downtime due to mistakes I/my fellow developers make. Datastore latency is something of a painpoint, but it was something we knew going in and designed a bit for, and it also helps quite a bit that we're very read heavy in the ways that appengine wants you to be.
My friends at wolfire ran the humble indie bundle (and lots of other stuff like their blog as well), and it does well. (documented at http://googleappengine.blogspot.com/2010/06/how-app-engine-s...)
Is a great platform for all the same reasons Heroku is a great platform: scalability, easy deployment, no sys admin.
Also, the upcoming "business class" service offers an SLA so it appears that the platform is maturing nicely. Since the recent datastore fix performance has been superb.