Pros
There are a wide variety of projects and very knowledgeable people. There's a bit of flexibility and good work-life balance. The company offers strong benefits re: professional development, conference attendance, bonuses, stock, etc. The company isn't so much like many other research organizations that lay off staff once projects end--since there's a consistent flow of contracts, there's almost always some project to provide you coverage. However, this is a good place to start before moving on to graduate studies or some other position--Westat is a respected organization.
Cons
There are definitely high peaks of work around deadlines and proposal writing times, but there are many lulls in work that have made it difficult to remain engaged. Though there are many projects, your ability to work on different projects depends largely on your supervisor.