I love FermCap-S. It helps prevent foaming and boilovers by affecting surface tension. It also keeps the krausen under control (don't need as much headspace)--no wasted beer in blow off tubes, no clogged or blown airlocks. I'm able to easily brew 4.5 gallons in a 5 gallon corny keg with a fermenter lid and 3 piece airlock. The same is true for my plastic fermentor buckets. The "high-water mark" ring of trub in the fermentor is no more than an inch or so above the beer. I do brew in a temp-controlled fridge which does help keep the fermentation under control, as well.
I've not noticed any detrimental effects (e.g., beer foam height or retention, lacing, flavor, aroma).
You can still get boilovers even with FermCap-S. Two situations come to mind:
1.) Boiling your starter wort in the Erlenmeyer flask--really watch the wort as it gets close to boiling and reduce the flame/energy or move the flask off of the burner ASAP (while wearing protective oven mitts on both hands, of course!). I have a ceramic stovetop, so I sometimes move half of the flask off the burner to help control the rate of heat transfer as we approach boiling.
2.) Bringing your first runnings to a boil while sparging. The first runnings are more concentrated (higher SG) and more prone to boiling over. It is easily neglected while your attention is directed towards collecting the second runnings. Be sure to add the FermCap-S right away and transfer some of the second runnings ASAP to help dilute the first runnings (prevents scorching and boilovers, IMO).