BeerBorg

Ive never used it
And Im not real sure what its for
What are the benefits and are there any drawbacks?

It's an anti-gassing ingredient. I use it when making yeast starters in my flask on the stove and not have a boil over. Many use it to reduce the amount of krausen in primary fermentation and others to prohibit boil overs during brewing.
There are some concerns by many on it's use and Fermcap-S is widely used. There are other alternatives but just about every beer forum has discussions (and arguments) about it's use.

I use it to help stop boil overs when the volume is too close to the max of my kettle.
It works in a fermenter the same way to help stop an overflow. it is the same material that is in Gasex.
I've had no negative effects from using it ( except dropping the dispenser into my fermenter).
I've read that it flocs out with the dropping yeast and has no effect on head retention.

"Designed to control foam development in the boil kettle and during fermentation, Fermcap – S reduces surface tension of wort and fermenting beer, preventing boil over and volume losses respectively.

Food Grade and removed during fermentation by yeast and later filtering operations, the active ingredient Dimethylpolysiloxane is a milky liquid added prior to the beginning of the boil or cold wort following chilling operations at the rate of 1 – 3 drops per 5 – gallons. To maintain Fermcap’s effectiveness, store between 5 – 20 degrees Centigrade and DO NOT FREEZE."

From the manufacturer.

Since you asked about drawbacks - well this is the only one I know - In a commercial brewery that uses it, it is filtered out of the beer and/or eliminated when they brite tank it - and as far as I know it is required to be by the FDA. Although considered food safe in small amounts, some folks do not consider regular consumption of the stuff to not be safe in the amount you'd get from drinking beers every day if it happened to stay in the beer. It may or may not - you can hope it ends up in the trub or in the break in the pot. It might. Or it might not. It would depend on many factors. Or you can just not worry about it, and probably nothing bad will happen.

That said I won't use it, I just try to brew a batch of beer that fits in my pot, and worst case I just spray the foam in the pot with water from a spray bottle to knock it down and/or use my little knob on the stove to turn down the heat, both of which are also effective during the boil. And I leave lots of headspace for batches that I think will blow the lid off.

To expand on Mashani's comments....the reason that it should be removed prior to consumption is that there is silicone in it. I believe that is the big argument on why you should not use it. Those that like it and use it argue that there is no way that your ingesting a detrimental amount of of silicone. Notice that in the explanation by the manufacturer that Banjo-guy posted it says: "removed during fermentation by yeast and later filtering operations"

I don't use Fermcap either, but I do use a silicone free anti foaming agent in my kettle to reduce the foam during the hot break. I add drops as needed (usually about 10 drops or so) on top of the foam, then stir it into the foam. I've never used in the fermenter though.

Here is what I use: http://www.chicompany.net/index.php?mai ... 95650a55de

I user it for almost all 5g batches to prevent boil over. I am not really all that concerned as it only takes a drop or two to drop down the head. I generally do a secondary as well (not for the Femcap reason) so I have plenty of time for it to drop out. Have never had a problem with head retention on my beers so figure it is dropping out just fine.