Pros
By their nature, large software consulting companies are difficult to assign a 'personality'. Since they don't have product lines, these companies tend to market themselves via their consulting methodology. IBM, for example, likes to sell itself as a cheap, global consulting company. Thoughtworks, as the small but cutting-edge boutique alternative. CGI's identity is based on process. Since they can't be the cheapest, they attempt to be the 'best run'. There is a focus on performance rather than minimizing cost, which engenders a more professional atmosphere than at some other companies. In general, CGI respects its employees work-life balance and provides competitive salaries and benefits.
Cons
The flip side is that the focus on process is often only skin-deep. When talking to clients, CGI likes to tout its ISO-certified project management and HR processes, but in practice only the higher levels of management concern themselves with process. The project teams manage themselves no differently than any other company. In addition, like any consulting company, your experience as an employee can vary a lot depending on what projects you are working on. Most of my time at CGI was spent on the client's site, and CGI made little effort to engage me beyond sporadic performance reviews. It would be easy to become lost in a company like CGI.