Pros
Lab49 is an exceptional group of people. I met some of the most brilliant yet completely down-to-earth software engineers there. Everyone is deeply technical, no PHB managers. You'll be surprised when you get stuck on a tough problem, and someone with an Associate Director title happens to walk by and show you real quick where exactly you have to set the breakpoint to diagnose your problem! If you want to advance your skills and work with people from whom you can learn Lab49 is the place to go. I was also surprised how cutting-edge most of the projects for the large investment banks are (I guess technology has become the main differentiator, so everybody is trying to utilize the latest and greatest). There seems to be a moderate bias towards front-end solutions (lots of JavaScript technologies), but plenty of interesting stuff happening on the back-end as well (Java 8, Scala). Great office on the 50th floor with conference rooms overlooking Central Park, need I say more?
Cons
No major cons, really. The reporting structure can be tricky, though, due to teams forming and breaking up frequently, as the project durations require. You could find yourself working with an on-site technical lead who reports to your deputy practice head manager (who has no visibility into your salary) who reports to a practice head (who has the full picture, but might only have met you once or twice). It's a common problem in the consulting industry, but it's a small price to pay for working with highly skilled people on extremely challenging projects.