Comparison between Exadata and Netezza Twin Fin. Ok, it comes from Netezza and as such is biased, but still an interesting read.
It is worthwhile to remember though that Exadata is designed for mixed workloads (OLTP and Analytics), which is a key differentiator to any of the other DW appliance vendors.
Interesting posts by Curt Monash on this

Thoughts on the integration of OLTP and data warehousing, especially in Exadata 2

Two cornerstones of Oracle’s database hardware strategy


Where is the response from Oracle?