Pros
There's a good chance you'll work on some big clients, which is a great resume builder. You get to see how other sides of the media business work outside of search and social. Work/life balance is good only if you don't care about being guilted into staying late (and if your team is somewhat competent). The job titles are also severely fluffed up at least one or two levels, which fools a lot of recruiters. But that can also be a con (see below).
Cons
In 2 years working in the NY office, I can honestly say that Catalyst is the most poorly run company I've ever encountered. Senior leadership has no direction and is making it up as they go along. They merged all Group M search and social with no plan in place, which led to understaffing, mass confusion, budget cuts, and caused anyone with any sense to jump ship. Doesn't help that HQ is in the middle of nowhere in the outskirts of Boston. The job titles don't mean anything. They hire people as "managers" with less than 1 year experience. A tip to the recruiters poaching from Catalyst - if the person started out as a manager and never got a promotion, that means they were entry level and never got any better. There were a lot of those people. Also ask them if they ever actually managed anyone (answer will be no). This leads to "directors" (and sometimes even their bosses) stuck doing menial work that associate level people should be doing.