Pros
Excellent opportunities for professional growth, client contact is frequent and early in your career, good balance of analytical and technical skills and communication/presentation skills. Great corporate culture, very friendly and casual (sometimes too much, I don't think its ever acceptable for senior management to wear cargo shorts to work!). Interesting work, and GREAT people there.
Kontras
Like I said, casual attire makes people feel like they aren't at work, and the professional atmosphere a 'consulting' firm should have is often absent. Another potential downside (we'll see in the next few years how this plays out) is the increased integration with the parent Nielsen Company, which is beginning to restrict many of the freedoms BASES analysts have been given in the past (for example, we are no longer allowed to use the Reply All button, and we can no longer give formal positive recognition to our co-workers without management approval). Small things like this are steering a great corporate culture in the wrong direction I believe.