Well, it has slippage in the sense that there is a spread and that the order book might move in the timeperiod from you send your order to it arriving at IB. But that's really the nature of an ECN - whether it will be better somewhere else, I don't think so.
If you have a decent size account there are other options, but all in all IDEALPRO is pretty good for forex. Dependes on the pair of course, as well.
