Marvin, if you are talking about other retail Forex brokers, I have experienced bad fills. Not with WS as the worst you can get is a re-quote, and one has a choice to reject that offer. In fact a re-quote is to be preferred instead of a bad fill, the latter will put your position in a very bad situation.
Well if you win at FXCM for instance, by taking advantage of lagging quotes, they will certainly not like that at all. The speed of the feed is what matters, and very few, if any SB's are as fast as Worldspreads. If a SB has a fast feed there isn't much to take advantage of, resulting in lesser re-quotes. So it is my belief, that a fast feed is in the long run a win-win situation for all concerned, even if you are missing out on those easy wins due to lagging quotes.