I’ve been using this mount in my Mac Pro for some time now. At $24.99, it’s reasonably priced for its excellent design and all-metal power-coated finish. It is easily the best mounting solution yet for installing a 2.5" drive into a Mac Pro (e.g. an SSD or laptop drive).
The sled bolts the drive in securely and allows far superior air flow to any other design I've seen, including Apple's own and the (yuck) Icy Dock.
With these sleds, you can install up to 4 SSDs or 2.5" hard drives in the four standard drive bays. The sled allows a direct connect of the drive to the motherboard SATA port, so there is no intermediate wiring, a big plus for reliability.