my favorite part about people from mckinsey has been that they're stunningly incapable. need some quick research done? after a lot of hand-holding, you'll be given an uninsightful and dubiously comprehensive summary of the mainstream view on the topic of your choice and a pivot table you didn't want or need, roughly 10 days after you needed it. you won't need to proofread the report, of course. but that isn't what you cared about anyway.
need something obscure and complex deeply analyzed and fully understood in an insightful way? again, you'll get some flashy combination of excel and powerpoint shit you don't need or ask for, maybe even ahead of schedule, but you'll also get an irrelevant speech about whatever tangentially topical prestigious person/institution the mckinsey person recently bumped elbows with. you'll also realize that the person working on the thing you wanted doesn't really have any understanding of the thing you wanted them to understand at all -- they're just regurgitating whatever they could find without analysis.
however, they've all been imminently presentable people who can articulate themselves clearly and appear to be very serious. this has been my experience with no fewer than 4 former employees -- all of whom were hired to perform tasks exactly like one would expect a business management consultant to be capable of -- but take it with a grain of salt anyway.