If I was unable to get the multiplier down so that our ultimate hosting cost is < $200 a month, I'd probably switch. But we have revenue to cover the hosting cost, so then again, it would be a big deal to divert or limited eng resources (me and one other guy) towards a big migration project. I'm guessing our hosting costs would be at least $100 a month anywhere else, and I've grown fond of a lot of the services provided with app engine.