Pros
FAST has a rock-solid development process, and their product is a shining example of how software should be written. You'll get access to the latest technologies and development tools. Benefits are excellent, and pay is well above average. The pay a flat-rate overtime rate, which is almost unheard of in the IT industry. The relocation package is very generous. The teams are well structured, and there is a great sense of accomplishment when major projects are completed.
Cons
Be expected to relocate to a different state (or country) every few years. It's not a good fit for large families, so the average age of the employees is about 27 years. Relocation can occur at any stage of employment, and can happen with only a few days notice. Be expected to work many long hours, or weeks at a time without a day off. The company has more than tripled in size in the past 5 years, so there are some growing pains associated with that. There are a few people that have been promoted to a management role that weren't really ready for the responsibility. Since their primary customers are State revenue agencies, the bureaucracy is what you would expect of any state government agency. Many of the people employed by the state feel threatened by the presence of outside consultants and are generally unhappy.