Pros
When SAIC was an employee-owned company, it was a respected and good place to work. You really felt valued as an employee. That changed when the company went public and eventually split into two companies. The old SAIC was known for hiring excellent people, and many of them are still with Leidos.
Cons
I worked on a new contract where the leadership was quickly staffed with newly retired colonels who never knew or were schooled in SAIC's culture of being a flattish company of professionals rather than a rigid hierarchy where decisions were made top to down, and after I left there were several ethics issues that were reported to corporate management. If you are joining Leidos, be sure to connect and talk with former employees via LinkedIn so you have a good feel for the culture of the project or task order you may be joining.