Pros
You can work next to or supporting some of the best artists in the world. The level of quality that comes out in every movie is inspiring. The majority of people are very friendly and nice. I always had the sense that people cared about my well being like a family. Some of the projects that you work on will truly push the boundaries of what you can do, but those are very, very rare. You can learn many things about the history of VFX since it honestly started there. Talking to people like John Knoll or some of the original Star Wars people is just a blast and is very interesting. The majority of the company is just a fan of what they do and that makes it fun.
Cons
Management will make changes in a recurring cycle in a way that shows they don't know how their company actually operates or where the money is going. It happens about every two years and typically involves the desire to drastically change how things are done. This approach is very disruptive and distracts the employees from fine tuning what they do better than anyone else - VFX. Management rarely listens to advice based on cost or ROI or any kind of analysis. It could be about ego, but I never felt that way. Not sure why they wouldn't listen. There is no career path. They don't know how to properly implement technology, even after hiring experienced Silicon Valley employees with solid ideas. They let them all go or forced them out. Disney cut deep and they are there for the IP. Can't argue with them. Most of the blame for where ILM will go has to fall on ILM leadership, though. Like someone else mentioned, don't hire your friends. The people responsible for hiring their friends won't ever own up the results of doing that, which is also a major, major problem there. Ownership and accountability. It's avoided like the plague.