Pros
Rackspace is many things, and will of course vary slightly from team to team. If you have the chance to get in as a Linux Administrator, TAKE IT if it's your first step into the industry or you need a break from the external norm. The 9-5 is respected here and even enforced. You have smart people all around you that will usually stay for at least a year, as well as a fun environment and fun coworkers.
Cons
Pay. The pay here is quite terrible for the industry as a whole. Rather than increase it they are simply hiring more people as more people leave for companies that pay competatively. Ticket crunching. The queus are increasing and headcount is always changing. Depending on your manager and department you may have to ensure you are giving it your all day in and day out, which can result in burnout if not careful. Low impact. Unless you work very closely with someone influential whatever projects you create will not likely see the light of day beyond your own repos. To their credit they have been working on this by offering rewards for good ideas that get implemented, but the program just started so we'll see how it goes. The good news is you are allowed to push to your public github depending on what you work on, so it's not too terrible.