The above is used for a desktop PC of a certain type/series and can be connected to any Mac via USB or Firewire.
A dock is a better solution if you have many S-ATA (2.5" and 3.5") HDDs flying around.
Enclosures are also good, but the constant swapping of HDDs with one enclosure might be a bit too much.
I have currently 6 2.5" HDDs in five USB enclosures and one Firewire 800 enclosure, then I have two 2.5" HDDs in FW800 enclosures and one FW400 enclosure for some other S-ATA HDDs I have.
It works quite okay for me.