Pros
Big company with lots of amazing people, small perks, decent on-boarding process, and opportunity to move around between groups. Each group tends to have a slightly different culture. Mine is phenomenal, and I feel like I'm surrounded by a bunch of close friends. We're able to agree and disagree openly, which is fantastic for engineering staff that sometimes needs to make difficult decisions quickly. The CEO is a call-it-like-I-see-it kind of guy. While this might be harsh or a shock for some people, I would always rather have an honest a difficult conversation than be left in the dark or not given details I need to make decisions. The company's core values, as lame as that might sound, are solid. It's not uncommon for a company to choose buzzwords as their values, but what's rare is to see a company that actually takes the time to define what each core value means to the organization and how employees are expected to uphold them at all times.
Kontras
Tasks and team objectives can move quickly, and some people just can't handle the context switch. This isn't a "con" for everyone, but if you want to work in a static environment from 9am to 5pm, you probably won't enjoy Rackspace. Also, if you're a minimum effort kind of person, you'll probably feel very out of place in the company. Everyone I've met works as thought they ran the company, and they take pride in giving Rackspace a good name. If you're not looking to do the same, you'll feel very left out.