Avantages
I may be a little biased but I have worked at FAST for a little over 2 years. My experience has been a pretty great one. First of all with a company like this you get to learn by doing, within a couple of years I went from being a complete programming noob to a a very well rounded developer. You literally learn everyday. You aren't working on one boring thing for your whole job here. You get to create and be involved in projects.
Inconvénients
In two years I've seen a lot of growth in management and in the company. But they really believe in their product and it shows, they are passionate about what they do. I think it can come down to the way you view things whether you take that as a pro or con. Now, I'd be lying if I told you I didnt put in some serious hours or have had really difficult days but I look back and know I wouldn't be where I am if hadn't gone through those days and put in that work. Sometimes deadlines have to be hit and there are times to grind but I can tell they have really put a focus on providing a healthy work/life balance.