Up front: I'm one of the people that leads engineering in EMEA at McKinsey Digital Labs (MDL). So please recognise my biases here! I also hope this isn't creepy, but was browsing HN and saw your post! :D
A couple of thoughts on your points, and I 100% think reaching out to the community here is the right thing to do and hopefully I can give you a (biased!) insider's perspective!
* You're absolutely right on the smart people. I've worked at big household name tech companies in the Bay, done startups and worked at VC's: I've never worked with a group of smarter or more capable people. If you're like me and enjoy being challenged, then MDL is a fantastic place for it.
* The crazy hours. That's a bit of a trope about McKinsey outside the Firm, people telling war stories that are often exaggerated. Do we work hard? Sure, is it any harder than I worked when working at BigTech™ in the valley? No. I am also in my 30's and it doesn't detrimentally impact my life.
To you questions:
* Learning - a big plus for me and the reason I joined. Have worked on more interesting tech problems and non-tech problems in my three years here than in my previous 10 elsewhere.
* Personal growth - obviously depends on what your goals are, but I would say it's a good fit. You can become a partner at the Firm as engineer, if that's your thing!
* Earning potential - very good.
I would say however, there is some travel. So bear that in mind.
If you want, reach out to me and I'd be happy to connect you with a couple of people in similar situations who can give you first hand experiences.
PS the 'up or out' thing is not in any way related to MDL.
PPS edited for formatting