Ventajas
Abundance of work. You don't have to worry if the program your on gets cancelled or paused due to funding, you simply get moved to where work is needed. Standard benefits. Having standard benefits is a plus these days, because lot's of companies have sub-standard benefits. You get your 4 weeks a year, although you have to accrue it. Opportunity for travel if that's your thing. Lot's of programs will allow travel around the USA and the world. Working on the bleeding edge of technology.
Desventajas
Typical defense giant. Some people end up on programs with bad management and tight budgets that don't promote. Some end up on fast tracked programs. Government work will slow you down because there is a process for everything. Things that you would think should only take minutes to do end up taking hours or even days. Such as having software installed.