Moved from Slicehost (after they were acquired by Rackspace) to EC2. Check out the reserved instances offered by Amazon for EC2, this brought the cost down to compete with the Linode and Slicehost but with a more powerful machine on EC2.

Reserved instances: http://aws.amazon.com/ec2/reserved-instances

I found the CPU time for micro instances to be sparse and unreliable to host an API for example.