Pros
Citadel is a company that strives to have every employee be their best professional self, and is absolutely true to its values - integrity, winning, meritocracy, learning, and extraordinary colleagues. The culture is centered around performance - there is an obvious, stated mandate that everyone perform at their peak capability 100% of the time, while constantly stretching that peak as much as possible. This is not your traditional finance company culture of "facetime" whereby those who are visible at their desk 24/7 are promoted and praised. Instead, those who contribute, exceed their goals, and bring the whole team forward are considered the top performers. And, of course, top performers are rewarded accordingly - not with badges of pride or soft praise, but with concrete results. If that's what you're looking for, Citadel is the place to be. From a technologist's perspective, there are infinite opportunities to learn from your peers, many of whom have different backgrounds, or completely different skillsets to offer. You may find yourself working with former nuclear physicists, or former fed chairmen; each interaction is a chance to level-up each other, and to learn something new. Of course, there are the perks: breakfast and lunch available in the office, stipends for teams to hold events, frequent company-wide outings (read: sporting events, broadway shows, etc.), and top-notch comp.
Cons
As with any company in an expansion phase, there are some growing pains. That said, those at Citadel have been less than at companies in similar phases. There are active efforts to mitigate bureaucracy, but occasionally they fall a little short. Given the performance oriented culture, if you don't make the cut, there is no time wasted before you are shown the door. Though depending on your perspective, this may not be a con.