The SSD EBS claims a default performance of 3 IOPS/GB with a burst of up to 3000 IOPS with a 99% consistency. One of my larger instances uses SSD EBS and so far I am happy with it. In particular, I was very impressed how much faster updating Ubuntu was when I first booted the instance. I am planning to move my Postgres RDS instance to SSD EBS next week. I think (I hope) it will fix a particular stall my application experiences sometimes.