Vantaggi
The company itself has a very strong future in government and health services contracts and they do their best to retain you and find you a new position if they lose your contract. Company covers cost of certifications, trainings, conferences, etc. Generally, you will work onsite with your customer - this can be either a pro or a con. I've had the good fortune of working with some very amazing military units and it was a very rewarding experience.
Svantaggi
There can be some irritating minutia such as multiple timecards, expense reports that are a pain to fill out and that you get little guidance for. The main con will depend largely on the customer that you contract for though. If they don't value contractors or have a lousy culture you probably won't have a good time. GDIT is usually willing to find you a job that's a better fit, but sometimes they don't have a lot of work in your local area.