Started with info session for summer internships for APD's (advanced professional degrees, e.g. PhD, MD, JD etc, but not MBA) on campus late November-early December. Applied for the summer internship and got invited to 1st round, IWIA, at the local office which involved the PST (Problem Solving Test) and 2 group interviews. PST was 26 questions that you had to answer in 60 mins. Lots of info available about it online. We were told that we will just be evaluated for the PST and that group interviews are for practice purposes. However, once I passed the PST, I was assigned a buddy and she was able to provide me w/ feedback from those sessions, and you can ask them for more details. The results were given 2 days after the test and I was shortly invited to 2nd round interview at the regional office (e.g. Chicago for all midwest locations). It was ~3 weeks between the 1st and 2nd rounds and they shared the contact info of everyone who passed so we could practice together. Also, at this point, you can finalize your office selection (up to 8 offices). They also held multiple workshop through Cisco webex. One was a group case interview and there were some where different offices tried to have you choose them in your preferences. You can also ask for a phone interview with your buddy. Sadly, my buddy did not do one with me.
Round 2 was two 1-hour interviews. 15 mins personal experience (PEI) where they ask you questions, sometimes related to your CV. Do check their website, b/c they ask you questions mainly from 3 categories: achievements, leadership, and challenges. You can easily find most of the questions on their own website and other places online and it's good to have 3-4 stories for each category. Then, you spend ~30 min on an interviewer-led case. Again, detailed info available online. At the end, if you have time, you can spend 10-15 asking questions from them. You should know that these last mins are as important as the rest of the interview.
The only negative part of my experience was how much I needed to wait to get the response for the 2nd round and at the end I was not passed to 3rd round. We were told that we should get the results anytime within the 2 weeks after the interview. All my friends got their results (positive or negative) 1-2 days after the interview. However, in my case, they missed their own deadline and finally I had to email the recruiter for the results. One of my interviewers called me 3 days after that. Be sure to ask them for detailed feedback in case you are applying to other firms. The feedback you receive for round 2 is the most valuable.
Finally, we were also told during the info sessions that 3rd round is just like 2nd round but with 3 cases as opposed to 2 and with more senior people (principals, partners, or directors) and would be at one of the offices you chose.
For me most of the experience was positive. Their HR is a force to be reckoned with. They make you feel like a part of them as you go through the process. The only negative experience for me was to wait more than 2 weeks for a rejection. However, all the consultants were very nice and especially my interviewer who returned to me was very helpful. I feel like you learn a lot just going through the interview process. However, if you get rejected once, they recommend that you at least wait 18 months before reapplying.