Forrester has just published (May 20th, 2011) a very positive research note titled "Oracle Exadata Raises the Bar on Database Appliances". Based on sixteen Oracle Exadata customer interviews (AFG and Sogeti are profiled in the research note), Exadata "exceeds performance and HA requirements", and has become "a key differentiator for Oracle", and "Exadata should be part of your database strategy".
(Note Dec. 2012: Full report is now NOT freely available. See Forrester site for purchasing)
Note: This research note was not commissioned by Oracle.