I bought two Fermonsters and after about 15 batches, one of the lids cracked and is now unusable. It loses one star for that. As far as folks having issues with the rubber stopper not staying put, you need to dry the stopper and the opening of the lid. If those are both clean and dry, the stopper sticks very well. The o-ring is another issue for some folks, but I just leave mine in place. I always soak the lid in sanitizer before using, and I just leave that o-ring in place. Even after kegging a batch, I remove the lid, keep the o-ring in place, rinse it off well with warm/hot soapy water, then soak it in sanitizer (since I already have a batch mixed up if I'm kegging), then air dry and store it. It's a pretty good lid, just wish mine hadn't cracked.
I think the lid could be made a bit better. I have had 3 lids crack at the seem. I was even trying with this last lid to make sure I put the bung in before attaching the lid so I could push up on the back side to try to prevent it from cracking and it still cracked.
Anyone else having problems with their stopper staying put during high krousen(sp)? I used a number 10 stopper and inserted some tubing to act as a blow off but the stopper keeps popping out of the hole in the lid. I put the stopper in there as tight as I can but the thing does it every time. Tried a 3 piece air lock, same result.