Customer support of Google really sucks! Currently the GAE cloud has a reliability problem (also for new customers). Instances are restarted like crazy. This leads to downtimes. But that's not enough. Customers have even to pay more(!) instance hours because of this. There is the running gag on the mailing-list: "Whenever GAE is unreliable for weeks Google needed to make revenue targets ;-)"
References
Current Issue: http://code.google.com/p/googleappengine/issues/detail?id=8844
Same issue from last year that took weeks to be resolved (check last comments!): http://code.google.com/p/googleappengine/issues/detail?id=8004
Some Pros and Cons of Google App Engine in this blog-post: http://www.mosbase.com/