Pros
Fun atmosphere, constant change, lots of challenging work, and significant opportunity for advancement if you play the social and political game correctly.
Cons
There is a very dark underbelly to the company's constant focus on its core values. In most cases, people really live these values but they are very frequently used as a weapon to ostracize people who challenge too much or who are disliked by the powerful social cliques that run the place. Rackspace claims to be a pure meritocracy but it is a great deal more like high school. If you can be one of the popular kids and are good at what you do, you can grow an amazing career at Rackspace. On the other hand, if you are great at what you do but you are threatening to any of the in-groups, you're going to suffer intensely.