I know a couple members here use this in their blower. Is this something that is highly recommened to use if one is going to add some performance to their engine? Would it make a huge difference over the OEM coupler.

Also, what exactly is it for?

I think its just a good quality replacement if yours has gone south. Not that it will give you any gains over the oem version.

Should I only replace it if I hear noises from the stock one?

Not a performance mod, just a better longer lasting coupling over stock.
This is what joins the pulley snout to the sc gears. It wears over time and needs to be changed out at some point?
With the m62 you have to remove the sc to get at the coupler. With the m90 you only need to remove the snout.

If the SC rattles at idle, or you can take off your sc belt and turn the pulley either way (car off) you will want to replace the coupler.

Is this a common problem?

Reason I ask cause it seems like it might take a while to replace.

I wouldnt say common, it may in be in higher mileage cars. Others will have to chime in her on that one.

Most used L67'* will need a coupler.

Almost twice as much as an L27 :P

Around 100,000 miles is average for replacing the coupler. I just changed mine at 107,000 with a zzp coupler.