It was fairly standard. The entire thing took about a month from application to offer. About a week after I applied, I was scheduled for an informal 30-45 minute phone interview with an Associate discussing some basics (e.g., running over my resume/CV, my interest in marketing research, in MaPS specifically, etc.).
The next day, we scheduled an in-person interview for one of their offices. Essentially, this consisted of three 30-minute interviews. These were pretty informal--in my opinion, at least--and a lot of the content seemed to overlap. Prepare to talk about what type of work experience you have, your specific background using data to answer research questions, and why you want to work for MaPS (vs. other marketing research companies--e.g., ZS, Forrester, etc.). This is followed up with a case presentation. You're given a dataset and some exhibits to work off of. Your task is to present your recommendations to your interviewer about what the client should do. You will be grilled a bit, but it's not bad. It's definitely a lot worse at some of the bigger management consulting firms. Lastly, you get to shadow someone in the role you're interviewing for. The consultant I met was friendly, personable, and genuinely a great guy. He was real about the hours (like any consulting firm, 50+ is average and 60-70 isn't uncommon when project deadlines are approaching). The atmosphere isn't anywhere as scary as some of the reviews here have made it seem. It was relatively quiet, but the employees seemed to like each other and had some time to socialize during my visit. Junior colleagues (Analysts, Consultants) typically work with collecting and analyzing data along with preparing reports and decks for clients (not much travel and little client-facing, which were both positives in my book).
I was given an offer a week or so later by phone from a partner.