I have the Guardian Maximus and it works pretty much as advertised. Plug it in and as far as your computer knows its just a normal external drive. Not sure about the Mercury, but the GM has USB, FW400 and FW800, though speed will be more reliant on the hard drives than the cable. I just like not having to use up one of my USB ports and actually have something use the FW800 port.
However, like stated above, its a difference between performance and protection with the OWC. I use mine as my Time Machine backup and storage for stuff I don't need on my laptop drive, so speed doesn't really have a huge impact. However, if its a drive that you'll be reading and writing to, speed might be more important.