It’s supposed to reduce blowoff without effecting head retention. Your thoughts.
I used to use it, until I found out that it plugs up your kidneys slowly over time. 
Now I carefully select the primary in consideration of all the factors that might lead to blowoff. I use carboys when the probability is low (lagers), and an oversized bucket when the probability is high (hoppy, high gravity, type of yeast, etc.).
There was a fairly comprehensive discussion about the potential health impacts of Fermcap awhile back.
http://www.homebrewersassociation.org/forum/index.php?topic=7419.0
I use it at the recommended dosage when I’m worried about blow-offs. It works pretty well.
I’ve heard that alcohol slowly hardens your liver over time, but that hasn’t stopped me from drinking beer.
I share Chumley’s health concerns, but being stupid I continue to use it.
I am a fan of FermCap-S. The kidney claims are unsupported and alarmist, IMO.
I use it in the boil kettle on every brew. At the rate of a couple drops in 10 gallons, it does not worry me in the least.