Pros
I worked in the Glasgow office as a developer for a number of years, starting back when there were just a handful of people. It was an amazing centre run by some great people and I thoroughly enjoyed working there. The ODSC are modern, flexible and will bend over backwards to make things better for those who work there. It's just unfortunate that they have to live within the CGI world. The biggest pro is the people in the centre. Some very talented people to learn off of, and some great personalities to make your working enjoyable. There's plenty of stuff being organised regularly so you have lots of opportunities to mix in and make friends. If you can get into the ODSC on a decent project, and you're happy to do the minimum to keep corporate out of your life (do your timesheet, answer emails, etc) then you could have a brilliant time there. Best people to work with doing great work. Pay is good as a new joiner, but raises will be terrible and you'll fall behind the industry average. My ratings at the end of this are aimed at ODSC, not CGI.
Cons
CGI. A behemoth company that refuse to move into the modern world. Their biggest concern is profit which isn't unusual for a company but other companies also care about employees - CGI don't really. Bonuses are almost a myth now, projects get micro-managed into oblivion, they refuse to budget enough money for realistic pay rises yet find enough money to hire contractors at 4x the rate of the people leaving. God help you if you forget to do a timesheet - but if you do remember to do it, don't record your overtime or the company will get mad that they have to acknowledge they aren't paying you for your extra time.