Pros
ManTech is a large company supporting government contracts in various industries. They're an established company with solid policies and procedures to guide you in performing your job. The opportunity for professional development is impressive with ManTech University which provides opportunities for credit - particularly if you're a Project Management Professional (PMP). The people are overall good to work with and are professional.
Cons
Since the company supports contracts, there's little room for upward mobility unless the contract is expanded. Essentially you're hired on for a period of performance to do work on the contract within your respective field. The company has been changing it's organization and is subject to it's success in winning contracts from the government - who at the time have little funding. This is driving them to cut overhead and reduce cost to remain competitive.