I've used SA several times in the past - all over this blue sphere. Like anything, be sure to read the fine print - those service/up charges can add up. And, be sure to know the marketplace before getting caught up in the bidding euphoria. Be patient, there's always another one right behind. I refuse to bid more than about 60% of lowest rates found elsewhere. If it goes higher than that, somebody is running up the bidding stupidly.
That said, NEVER had a problem getting dates I wanted within the offered window. In fact, they guarantee (or did, have not used SA in over a year) if you cannot get your dates - the deal is off.