Rackspace user here. Be prepared to pay $$$$-their hybrid environment is not cheap, especially for servers with above normal specs. There are also (industry standard) restrictions on the network side of things with their VM's, which we weren't prepared for when we started. Lastly, if you're ever the target of DDoS attacks...good luck.
Previous Rackspace user here (we got fed up with Rackspace and finally got our own equipment for a song compared to our monthly hosting costs with them)... Yeah, good luck if you get a very common DDoS attack while on their network. Apparently their data-centers in general are large DDoS targets. We moved and have not had an attack since (and it's a big deal since we do e-commerce).
That said, out of any company I've ever been with their support was really good. Just understand you're paying like crazy for it.
