I've interviewed with all top consulting firms except McK, and BCG cases are much tougher than that of any other firms. BCG cases tend to have a focus on strategy - it is not enough to be able to follow through a framework and do the math correct. For one thing, BCG almost never gives you data right away. If you are asking, what are the customer segments, you better be prepared to give them some ideas as to how you would segment the customer. They also expect you to have some intuitive knowledge about competition and high level strategy.