Their customer support is shit, takes days to get back to you -- then without any real explanation, "Our engineers were able to fix the issue that our cloud server engineers caused."
Worse, let's say you have them look into your actual code (as I've seen some clients do), and 1/2 the time Rackspace just outsources the work to whatever crappy dev shop has availability. So then the followup ticket is, "Why are all the permissions on the server jacked up?" And the response, "Oh, we didn't know how to run the provisioning scripts, or read the documentation, so we just set everything everywhere on your production box to 777... cool?" RUN away from Rackspace. Run.