I am currently a customer of Rackspace Cloud (Cloud Servers to be specific).
I am very happy with their service except that it gets too expensive under certain scenarios:
Their 256MB/10GB VPS is only $10.50/month but the problem is if you need some servers with more memory and less disk, along with other servers with more disk and less memory, you overpay because the only combinations available are "more disk AND more memory" - the price, disk, and memory moves up linearly.
I would like to migrate some servers over to a cheaper provider. I don't care if it's a bit less reliable because to be honest Rackspace is more reliable than I need, for some of the services that I run. One thing I'm not willing to give up though is the fractional billing... ability to create and destroy servers and only pay for the time used.