In 2008, shortly after App Engine launched, I was running an app that was a totally bad fit for it (tons of concurrent writes to the same rows). I actually talked on the phone with a Google engineer to try to figure out how to optimize the problem. That was pretty sweet.
On the other hand, I was managing the community for that same project through a Google Group, and one day the group simply disappeared for 24 hours or so -- it was as though the group had never existed, but couldn't be recreated. Then it popped into existence again, no explanation. There was absolutely no way to even ask for support, let alone get it.
My takeaway, was, I guess, the obvious one: if you're relying on a web service for something important to your business, you need an SLA. If you're getting a service for free, you should assume it might vanish one day and never come back, taking your data with it. (Especially with a company like Google that's known to let old products languish and to have no meaningful path for reporting problems. ) Sometimes that's OK, sometimes it's not.