Pros
There are many talented, hard-working people at Rackspace and you may get to work with them. Some of the tech they work on is new and interesting, and working for Rackspace is (currently) seen as good experience on a resume. Depending on your team, you may be able to maintain a relatively sane work-life balance.
Cons
Rackspace has never been known for competitive pay and their raises leave much to be desired. The culture that people came and stayed for is now gone, thanks to the new CEO. Many benefits have been cut or removed (including stipends for teams to have events, spending on conferences and learning opportunities, and sabbaticals). There was also a rash of layoffs at the start of 2015 without any transparency as to why we were laying off hundreds of people, except that it was not related to job performance. Although I was lucky enough to have an exceptional manager, many do not - and middle management is as bloated as always with people who were not given a technical career track option and have no aptitude for management. Expectations of managers have continued to be poorly defined (should I be a people manager, a technical manager, or an individual contributor?) and there are very few who can accomplish all three objectives well. Politics are absolutely rampant now - promotion decisions are made in bars and hallway discussions and do a disservice to the company by continuing to promote people who are better at canoodling than running an effective organization. Work-life balance depends entirely on your team and manager. There are a lot of incompetent and/or lazy people who have made their way into the company during previous growth phases and Rackspace does a poor job of asking those people to leave. "A" players are often forced to work the extra hours to make up for their lackluster team mates in the spirit of "fanaticism". Truly fanatical Rackers have burned out and left.