I have a Promise Pegasus R4 8TB configured in a RAID 10. After 1 year I have had 2 drives show up as dead. The first drive I ended up reformatting and zeroing the drive and it seemed to fix it. I am now doing the same with the second one. For the cost of the unit, and the fact that it runs as RAID, you would think that it has Enterprise class drives that are meant for 24/7 RAID/Archival performance. Nope, they are desktop drives that have firmware that isn't optimized for RAID performance.
With that being said, as a storage device I haven't had trouble with the unit aside of the drive failure. It seems to run/copy files fast enough even through our gig-e network. Make sure to keep the firmware updated.
I'll give it a solid three, if it had enterprise class drives I would give it a 4-4.5.
I figure they are buying the drives in bulk so the better drives shouldn't cost that much more, and I think it would pay for itself to Promise by reducing warranty claims.