Pros
If you end up on the right projects, you get interesting challenges and the chance to learn a lot about dealing with large cross-disciplinary teams and messy problems, and even as a recent graduate you can have a really big influence on major products—if you end up on the right projects. You work with some great people, and everyone is really friendly. The design studio (and by extension designers) currently have relatively strong political backing and a lot of pull within the company, so in some cases at least, you get to drive strategic discussions and sit at the table with product managers, dev leads, etc. A lot of big companies are trying to make design work for them right now, and IBM has one of the most serious efforts in this area, so you can learn a great deal about how it might work and how to go about helping an organization become more user focused. A lot of what determines your experience also depends on your attitude and expectations—if you look at challenges as something to learn from, you will learn—if you want an easy job where someone has already figured everything out... You will be disappointed.
Kontras
A lot of teams are quite dysfunctional, sometimes for reasons internal to the studio, but usually because of the widespread bureaucracy and politics within the broader company. Also, lot of people choose to work at IBM because they can hide in the vastness and get away with doing next to nothing, so mediocrity and lack of motivation are the norm outside of the design studio. A lot of people are completely out of touch with current trends in technology and design, at least as far as user interface stuff goes. Another downside is the fact that even within the studio, the real goal is just getting to a place where design best practices can be applied, not pushing the field of design forward. There is no need for real innovation or flair, just for applying techniques that were worked out ten years ago to problems defined years before that. While what IBM Design is doing can be quite exciting, there are a lot of exciting things happening at the intersection of design and technology right now, and a lot of them are much more exciting than IBM. Also, expect to be around a lot of people just out of college with little to no perspective.
