Pros
Some good people and some interesting projects, full-stack development with everything from UI to back-end high volume message processing. A good place to learn about the finance industry since all kinds of companies trade on the exchanges and clear though the CME. It's a very profitable company that pays accordingly - bonus, shares, generous pension. Work hours are reasonable - 9 to 6 Monday to Friday , with a fair amount of flexibility.
Cons
The CME makes a huge amount of profit through what appear to be monopolies on certain marketplaces. These keep ticking over without the need for much innovation. So there isn't much incentive to eliminate "dead wood" from the payroll. Over time I think this has led to a gradual accumulation of ineffective managers and incompetent support staff who would have otherwise have been moved on. London is very much a satellite office. This is fine if you're working on a properly devolved project with local leadership, but it can become painful if the US is directly in the driving seat. It often feels as if you're brought in at the tail end of important discussions when all the key decisions have already been taken.