They only vanished once (the entire bid and therefore the entire cluster), and I think it was actually due to a glitch; the price never actually went over our bid. That's the only time we ever lost our spot instances. I think we actually bid over even the normal (non-reserved pricing), because I've (almost) never seen prices cross the reserve price, and even if we had to pay full price for a day, that's fine -- it's still tremendous savings in the long run.