Phoronix pits a 12-core ARM cluster against Intel and AMD processors, you can check the results over here.
"Last week I shared my plans to build a low-cost, 12-core,
30-watt ARMv7 cluster running Ubuntu Linux. The ARM cluster that is
built around the PandaBoard ES development boards is now online and
producing results... Quite surprising results actually for a low-power
Cortex-A9 compute cluster. Results include performance-per-Watt
comparisons to Intel Atom and Ivy Bridge processors along with AMD's