Pros
There are several great projects, and plenty of opportunities, so unlike many consulting companies, you are rarely "on the bench" for an extended period of time, and you have several options if you want to move to another project. There are a lot of great people to work with and learn from, and the work/life balance is great. CGI is a great place to begin your consulting career.
Cons
CGI, at least in the Federal sector, has a high turnover rate, and in my view, there are several reasons for this. For one, the company does a very poor job of providing training. Learning as you go has its benefits, but also its limitations. It also is known to pay slightly under market, and raises can be rather paltry, with no bonuses given. The company is notorious for giving promotions without accompanying raises. The health benefits have deteriorated signficantly in the past several years.