We also use two DSS v6 up40 as iSCSI SAN for our Hyper-V R2 cluster (specs see signature).

Performance and reliability are good, but we have two issues:

1. DSS failover breaks Hyper-V VMs on cluster (cluster disks going offline/online and VMs are bluescreening).
2. After updating to up50 CSV goes to redirected IO because of issues with SCSI-3 persistent reservation. We did downgrade to up40.

We opened support tickets and are waiting for response ...