Pros
I've worked with some great people and done interesting and important work. I've mostly been left alone to do my work and have not had to attend many meetings. This allows me to be more productive. ManTech handles their money well; there is no running out of funds before the end of the contract period, at least not due to mismanagement on ManTech's part. And ManTech has made an effort to go paperless, meaning that a lot of the forms you need can be filled out online and are part of a formal, automated workflow.
Cons
ManTech likes to say they hire professions and let them do their job. in practice, this means that you get very little input, advice, or support. This begins with the InDoc process. I've work projects where I had no idea what the chain of command was. I'm not sure how ManTech interacted with their government client, but it seems as though the relationship was toxic, so much so that even though my project had several successful Go Live's in a row, the government still preferred the former Prime who had run out of funds and basically ran the program into the ground. The ManTech University is substandard, and the ManTech Knowledge Online is disorganized, with a lot of important corporate information (like formal processes and templates) unavailable. Perhaps this is due to poor search functionality; I can't be sure. Outprocessing is really bad; I have often been unable to locate the people who are supposed to sign off on my Out-processing.