I have used Fermcap-S in 50 batches of beer, and have never had any problems with it. In fact, it's the same bottle that has lasted for 50 batches, and may well last for 50 more.
I only recently started using it in the boil (last 20 batches or so) and was pleased to find that it was doubly useful. I've also had success using Fermcap-S to knock down the foam when making maple syrup, although I had to add it several times.
I really like the product, and highly recommend it to anybody out there. It works great in wine, too!