I was nervous about those in fruit fly season tom....??Why? The edges all seal very well and don't draw anything into the carboy. If a fruit fly can get through one of those then they can get past a solid stopper.
I have a couple of these (purchased on Toms recommendation, thank you Tom) that I use on my sour carboys. I don't worry about flies or othe nasties getting in but I guess I'm skeptical that the design really prevents O2 from getting if and especially after CO2 has been purged and therefore necessarily loosened the seal between the silicone ports and the cap. The cap is only held in place by a semi-snug silicone on silicone contact. It's design is such that going out is certainly a path of lesser resistance. I just wonder if reduced pressure inside the vessel due to cooling will really reset the cap sufficiently to prevent o2 from getting in.