I have used these since day one and have been sold on them. Never had an issue with them and the next morning after I have started the yeast, I love that I can smell the yeast so strongly. I have probably 20+ brews on one and still working great, but yeah a little yellow like started before me. But hey if it works, keep going, if it plugs get a new one.
After discovering that a cluster of fruit flies had crawled into my yeast starter and committed suicide (and me having to toss said starter, lest I end up with 5 gallons of vinegar), I stopped using foil to cover my Erlenmeyer flask. These stoppers solve that problem. They let in air, let out CO2, and keep bugs, dust and microbes out. Easy to sanitize--just boil in some water for a few minutes right before using. And at 70 cents a pop it's good to have several on hand. They do turn yellow and get nasty after several uses and need to be replaced.