Pros
I have been with FAST for over four years now and been assigned to both US and international sites. In that time, I feel FAST has given me opportunities for personal and professional growth beyond what I could have expected when I first started. The small project sizes (typically less than 30 people) make it easy to develop relationships with your coworkers, while the large size of the company as a whole (~1000) make it interesting to meet and learn from a variety of people. I feel FAST has given me all the room in the world to grow professionally and not only taken an interest in how I want to further my career, but given me the tools and opportunities to do so. They have addressed my concerns when I have had them and definitely cultivate a culture of "one big happy family".
Cons
For me, the uncertainty of knowing where I will be in the next year (or less) begins to grate after a while; it is difficult to make long term plans when you don't know where you will be living next until approximately a month before you move there. The long hours are stressful and wear you down after a few months of it. Additionally, as the company has grown, it has lost some of the personal touches that used to make the employees feel like FAST really cares about them as people. Administratively, there is more red tape then there used to be, which is difficult to swallow for older employees who are used to a different way of things.