Pros
During my time at CGI I've had the opportunity to work on a lot of interesting projects, the variety of work has been very motivating and also one of the major reasons I've chosen to stay with CGI over 15 years. The general culture is good to, although there are office politics and some weak managers around on the whole it's generally BS-free and a pleasant place to work with everyone focused on delivering an excellent service. The top management is very strong and have made some good decisions over the years that's enabled CGI to grow steadily and remain financially strong. The training on offer is very good to and it's seen as a key part of CGI maintaining a competitive advantage.
Cons
There really isn't as much mobility through CGI as you're sometimes led to believe, moving country seems almost impossible for example. Salary & benefits are based on industry average, which CGI seems proud of but to me I think if you offer the average then you attract the average. Post-Logica acquisition things have changed dramatically in Europe, virtually all middle management and most of the senior management is ex-Logica. I've gone from having a good working relationship with my boss, fostered over many years, to being lucky if I get a response to an email from my new temporary boss (and I don't know them well enough to say if it's just incompetence or sheer workload that's responsible, what it isn't though is good management).