Pros
Leidos is the "science" half of Science Applications International Corp (SAIC), when SAIC split into two companies. If you are a scientist or highly technical engineer looking for intense challenges and smart coworkers, look at Leidos first -- the other, less science-intense half kept the name SAIC, so don't be misled. I worked there for a decade and enjoyed almost all of it. Got to work on heavy scientific topics that I wouldn't have been able to do elsewhere -- at other companies the really fun science gets done by the few eminent PhDs, leaving crumbs for everyone else. At Leidos my experience was usually plenty of science. Sometimes more than I could handle. Projects are ambitious and challenging.
Cons
Chasing government contracts is a real drag. Bureaucracy of govt funding is getting worse, funding $$ drying up, and you wind up hunting for charge numbers to work for. Turns into a stressful drudgery after a while -- it can be hard to concentrate on your current contract while continually worrying about the next one. Job security is only as solid as that next contract. Work can be stressful, certainly is more than 40 hours a week. Tight deadlines can lead to lots of overtime.