Pros
CoStar goes a long way to provide unique perks (partly for the prestige it brings them, partly to help cut the painful attrition rate). They are making sporadic efforts to fix the issues that are causing so many to leave, and there are management staff in place who do go out of their way to make the experience a positive one - they are also beginning to promote from within rather than exclusively hiring managers from outside. They are a rapidly growing company that is constantly pushing the envelope for their products.
Cons
The last pro is also a con - because CoStar is constantly trying to do the newest and best thing, the disconnect between senior management and research/sales staff is huge. Contradictory information is given from manager to manager, metrics are changed at the blink of an eye, and you learn quickly not to rely on what you're hearing from management, because chances are it will change by next week. The pay-for-performance environment is made even more difficult to navigate by the fact that senior management can (and does) change parameters for minimum performance, bonuses and contests midstream. Even worse, dissent within the ranks is not tolerated - if you complain too loudly or to the wrong person, you risk having your bonus cut or your position terminated.