Pros
Most of your co-workers (fellow "rackers") are likely to be great people. If you work in San Antonio, the headquarters (a former mall dubbed "The Castle") is pretty awesome.
Kontras
Since going public, the sincere focus Rackspace once had on its customers and employees has diminished somewhat. The heavy emphasis on profit drives leadership to be much less focused on greatness and far more focused on spending less money in order to increase their bonus payouts. Working at a satellite office, such as one of their many data centers, is not at all like working at their San Antonio headquarters. There are fewer perks outside of headquarters, and the perks you do get may be reduced (e.g., infrequent or underfunded outings). Additionally, there can be few opportunities to move up or change job roles in any capacity without transferring to a position in San Antonio.