Pros
By far the best reason to become a Racker is the caliber of people you work with every day. This place has been able to bring in people that truly care about each other and aren't afraid to show it. If you thrive in a dynamic environment where change is expected you will succeed at Rackspace. This company is always evolving to better serve customers and you need to be highly adaptable to do well.
Cons
This company is always evolving, so if you don't do well with change then this company will not be a good fit for you. I have been with Rackspace for almost 3 years and I've had 6 managers without changing jobs or departments. There is a lack of procedures and processes that one might expect with a company of this size and age but in the time I've been here many standardizations have occurred and more are forthcoming.