I have had a Pegasus 2 R6 paired with a 2013 MacPro for a couple of months now - using it for video editing. The Pegasus is configured as a RAID 5 with 3TB drives. Thus 18TB of capacity gives an available 15TB of usable space.
The drive is a solid performer and I now have no issues with it.
Initially I had problems with one drive but after a session with Promise support, the issue was traced to a firmware deficiency which an available update fixed.
My only regret is not having purchased the R8 which would have given me the more feasible option of a 'hot spare' drive.
Due to those initial problems with one drive, I purchased two additional spare drives. It is apparently critical that all drives be identical. That possibly suggests that one can replace all drives at once. There is a list of 'acceptable' drives somewhere on the Promise Web site I believe.
The drive is quiet, fast, has very good power management and the rather complex software controller seems pretty solid and well thought out. The self-diagnostic & reporting features are good.
I currently use an older Sonnett eSATA 4 drive enclosure with 4TB drives as a backup to the Pegasus. I would not recommend this as the RAID 0 with 4TB drives is a little flaky. The OWC Thunderbolt enclosure would have made a better backup system I think.