Pros
Rackspace has attracted some extremely talented and intelligent people over the years, and it is a joy to work with them. As a hosting company that focuses on customer support for Linux and Windows servers, the system administration roles provide many interesting challenges. The 'support' role feels more like a consultant role in many cases. This can be frustrating, though, as the overall system administration of a customer server is left to a customer, and this can result in less than ideal systems.
Cons
Rackspace is not known for its compensation or promotion paths. The structure of the company can be confusing with multiple people or departments handling very similar responsibilities. It can be frustrating to propose changes or try to communicate with other groups, as it is difficult to know which groups to engage.