I bought the p400 about 5 years ago to house my gaming PC. As it got older, I transitioned to an ITX setup for gaming and started using my old PC up as a NAS/Server.
I wouldn't have picked this case if I had to pay for the extra brackets since I had no use for them at the time but appreciate being able to upgrade the p400 to hold 8 total drives without any case mods now.
The brackets have rubber bushings to keep the vibrations down and slot into place over the cutouts to the cable management trough. I like how they keep enough space between the drives to not impede airflow from the front fans.
The only small gripes I have are that the drives sit a few millimeters too deep to comfortably use right angle SATA cables and there isn't anywhere to store the plastic cutout covers when not in use. I'd pick 4.5 stars if that was an option.