Pros
Rackspace likes to maintain it's startup-like culture. Casual dress, free food everywhere, free sodas, fun team outings, etc. But most of all, I would have to say the best part of this company is the caliber of employees they hire. This includes not only their skills but also their personality. You can be a very high level expert in a field but they won't hire you if you have the social skills of a brick. Real-work skills and experience are important that are relevant to your position.
Cons
Don't expect to start out with a high salary here. You will have to earn your place at the company for the first couple of years, but after that you'll be rewarded handsomely. The one best mitigator is the large amount of vacation. We get 20 core vacation days on top of every single Federal holiday.