Pros
The single reason is the people there. When I first came in, I was amazed at how friendly and helpful everybody was and now that I've been here for six months, I'm still really amazed - this is somewhere in which everybody wants to share in the company's success! The products are very exciting and there is still enough of the start up mentality here that you can make a difference. RIM will probably double in size over the next year, so there are still a lot of opportunities for advancement or to carve out a career for yourself that fits your personality and skills.
Cons
There are some senior managers that are too entrenched in their positions and have grown somewhat bitter and cynical (if that seems possible). For these people, there needs to be some kind of re-energization process to be put into account. The place is growing very, very quickly. It is difficult to find a parking space when you come in and chances are your manager won't be able to find a desk for you. Finally, there is a lot to learn, I spent the first month there wondering if it was all a test (setting up my laptop, BlackBerry, system access, etc.) to see how well I could cope - this is very intimidating for many new employees.