My company had a dedicated server with Rackspace for over 10 years. I was in charge of the Hosting so I can speak to this first hand. Most of the time I was very satisfied, especially in the early days.
Customer service in the beginning was fantastic and it was just what we had been looking for. You could call or use the ticket system and Techs would get to you quickly, dig deep and solve the issue. As time went by (after they went public), service/tech support suffered. It started taking longer to get a response and the Techs weren't as knowledgeable on the issues.
It seemed to me that we had been assigned first level support Techs which was frustrating since we were long time customer and were used to a higher level of support. Honestly, that was the only reason to pay this much for a hosting.
It became more common to have to escalate ticket to a higher level and contact account reps to try and get issues resolved.
Not only did service suffer but they started nickel and diming us for everything on the server. This was getting more frustrating and expensive.
We had two very large customer on the server with databases. When they had separate issues, support told me they need to be moved to the cloud. We moved them over and it was a terrible experience for them (and me). They over sold them on cloud services and they wound up having more issues that went unsolved. They both left Rackspace cloud due to constant issues and therefore we lost two big accounts.
Speaking with them afterwards, they move to dedicated servers and were paying a fraction of what they were paying at Rackspace with NO issues!
Also, Cloud support is NOT dedicated server support. Good luck getting an answer in a reasonable time frame.
We finally moved away from Rackspace in August of this year.
In summary, I must say that Rackspace has lost their edge. Their old catch phrase "Fanatical Support" is a distant memory. They seem more inflexible that ever, controlled by stockholders and corporate profits. All the negative effects that go along with this is now apparent. They would rather lose a customer rather than give them a discount or incentive to stay while offering sub-par support. Ironic since it is what they built their reputation on.