I am a systems administrator, and have worked for large, high traffic sites for the last 10 years. That being said, I feel that the performance and availability of Rackspace Cloud has been great so far, and really love the options I have when purchasing servers for clients or testing purposes. Honestly, the network feels faster than AWS, the server feel snappier, and the price is much more affordable. I have not had to talk to support yet, other than for a quick billing question, since I was not sure about an item I not been billed for, and support quickly responded and let me know what the deal was.
In my career, I have been behind the phone/chat in support roles for small dots, and the largest computer company in the world, and even interviewed with RS many years ago, but when I look at their competition, I feel they are doing a better job, overall.
Amazon has never been serviced minded, although they try to play one on Tv from time to time (grin)