I think the in-person interview process is quite streamlined, and the manager conducting the interview clearly knew what he was doing. (Getting the interview was another story - next paragraph). I cannot say the same for other top strategy firms who give the interviewing task to 29 year olds with 3 years of work experience, an MBA, an an attitude problem. ZS gives you clear materials to work with, step by step (at least for round one) - it's not a case in your head where a recent MBA grad tries to play mind games with you on a power trip.
ZS seems to be growing very fast, and due to this, my application got lost in the shuffle for months. I applied about 6 months before my 30 minute interview. So, it's a little annoying that there was no point in 6 months that they could have just had me come in and do this, which honestly kind of points to their disorganization. I like the work that they do and really wanted to work for them, so I did keep contacting them (within reason) about getting an interview (they said that they would - repeatedly - but then they just kept pushing it back). On top of that, two recruiters were calling me & my information was being lost - it was a little disorganized and frustrating from the outside.
Finally, when I got the interview - I just blew the case completely. I knew it then & there... and that was the end (after waiting 6 months). Fortunately, I had a very nice and professional interviewer who made everything clear and had no ego or condescension when he saw my mistakes (again, unlike some other strategy firms with awful people). So, I would say this was a positive experience even though I was a train wreck. I wish I could do it again properly - because I had no idea what I was about to face, and I should have prepared for THEIR type of case which is nothing like McK, Bain, or BCG. It's very quant (8th grade math - but still - if you haven't done it in 20 years - not so easy) and interviewer-led.
I know I am just as smart and qualified (if not more) than most of the people who work at ZS based on my degrees and experience, but screwing up a case interview ends it immediately. There is no turning back. Again, it is also annoying because I find the people there very nice and the work interesting...but one case is the barbed wire fence.