I've conducted some independent tests of various block storage options with Rackspace, EC2, HP and GCE. Here are links providing a CSV of the results (may take a few seconds to generate). A value of 100 signifies near performance parity to a baremetal SAS reference system. Overall, Rackspace SSD is performant, but lacks the consistency of EBS PIOPs or GCE persistent storage.

IOPS http://app.cloudharmony.com/cloudgrade/api/query/cloudharmon...

IO Consistency http://app.cloudharmony.com/cloudgrade/api/query/cloudharmon...

There's also an excellent writeup by GlusterFS contributor Jeff Darcy:

http://pl.atyp.us/wordpress/index.php/2012/10/rackspace-bloc...

Here is a benchmark from the Rackspace site on SSD performance via Cloud Block Storage: (PDF) http://c744563d32d0468a7cf1-2fe04d8054667ffada6c4002813eccf0...