Pros
World-class talent, mission-oriented mindset, working "incidents that matter", good compensation & benefits. Having Mandiant experience on your resume lets you command a premium salary-wise when its time to move on.
Cons
The acquisition by FireEye caused a massive talent exodus due to a shift in business priorities (profit instead of respectability & renown) and pressure to cut down on overhead costs. An idiotic corporate travel policy enforced by FireEye & being constantly on the road combined were a major morale sap. Leadership & experience within the Professional Services component was gutted when several long-time Mandiant employees refused to push FireEye product on customers. MCIRT was neutered and hamstrung by bad/incompetent management eager to satisfy unrealistic demands by FireEye. Intel team gutted and demoralized.