I was invited to the first round of interviews, which consisted of two case studies with consultants, and one online case. The case studies went as follows: tell +- 10 minutes about your resume and answer 2/3 personal questions ('Why do you want to work at BcG?', 'Tell me a situation in which you managed to convince team members'), 20-30 +- working on a case, and +-5 minutes to ask questions. The online case took 45 minutes, and consisted of 23 multiple choice questions, all about one particular case. The test included tasks such as: calculate the ratio of product cost in situation A to the product cost in situation B, calculate the average gross profit margin, etc.
The people, except one interviewer, seemed very reasonable and nice to talk to. I think you have to be a bit lucky with your interviewers. Their judgments are the most important, and it is nice if you have a spontaneous, interesting interviewer that gives you some affirmation. I had one that gave me such little information about a case, that it took me great difficulty to give a reasonable answer.