Hi MTernesto,  I have seen one install using Twinax cabling; apparently there is only one specific type that works: Cisco Twinax REVB (part # SFP-H10GB-ACU10M10GBASE-CU SFTP+) The REVA version does not work reliably and causes performance issues.  They may have the same part number so be careful. (Thanks Luc!)  I did not see any problems with the cluster using the REVB cables.  Cheers,Matt 
I have heard from delivery people, that they had some bad experience using Twinax despite all the benefits from cost and power consumption from Twinax. Those workloads were block oriented mostly. intermitent connections were claimed. Regardless, most of the customers I've been to use Optical SFP+ connections in 10G.  Working on a POC with Greenplum DCA nodes noticed the interconnect uses Active Twinax Cabling and we had SFP+ from a Brocade switch to fit on that interconnect to connect our Isilon Nodes.  we tested writes and reads from the Isilon cluster and seems everything was working fine without any disconections that were stated.  Besides Length which can be an issue especially when connecting to workstations, I believe we can use them more in most of our workflows.  Yet, have you seen any noticeable difference in performance between the two types of connections?
Performance Differences between Twinax and SFP+ Optical links on 10G