Stoppers don't go all the way into the wine bottle.
921805
The bottle stoppers don't go all the way into the wine bottle and can fall out if you were to put the wine bottle on it's side
3715991
The opening is cut differently than the spouts that come with the bottles, i think thats why its so hard to get any liquid out. I tried to make a bigger slit but it just ruined it.
3550316
They fit easily but securely in wine bottles, but they are easy to remove - unlike reusing corks.  Also, unlike other stoppers I've had, they don't stick out so much that they are difficult to store in the refrigerator with the bottle straight up.
4100577
The opening for the bottle to sit could be bigger allowing the bottle to rest better in the shoe
1237576
A great value.  However, many of the holding slots are too wide for a standard wine bottle meaning they are held at less than an ideal angle.
860014
All silicone, this set of four stoppers are designed for wine bottles. The bottom diameter measures 3/4 inches and it flares slightly as it reaches the top cap so it snugly fits in a wine bottle. The cap measures 1 1/4 and the entire stopper is made of solid silicone so they are rubbery, sturdy and if desired, can be washed in the dishwasher. Function well.
2774565
These are very cute, but one of the bottles didn't have the "stopper" in the bottom.  Very disappointed.  Wish they had checked and been more careful.
3758627
The brush handle is a little too wide, making it difficult to get into small-mouthed bottles.
512813
Droppers don't fit bottles well. Seems like they weren't meant for these?
3470465
The price is good and works well as a siphon, but I really wish they took the time to make the check valve on the bottom a bit better so it can act as a better wine thief.
3939140
Though the wine racks look very cool in our country western bar, the boxes (bought at the same time) do not accommodate the same size wine bottles.  The depth difference of one of the racks really restricts 90% of bottles you can place in it.  My husband had them installed before we realized the difference, and no - I am not taking it down to order a new one, we will have to keep the limits of that rack in mind.  what a pain!
2235302
Very good product once i figured out how to use it, Stopper is a bit small for bottle so have to use some force to get it to fit
1893016
It works well with large double wine bottles or small ones too, it won't stay on if the drink is carbonated.
2885594
4/21/2017: I attached this to a 2 liter Kirkland brand olive oil bottle from Costco. Not the best use of it because of the way this bottle is designed as you can see from the picture. I will try to attach this to some wine bottle later and see if that makes it more stable. It is wobbly inside this bottle but did not see any leaks or drippings yet. The stopper works very well.

4/24/2017: Moved to a standard 750 ml wine bottle and it fits perfectly. No moving around when opening and closing the stopper anymore. As you can see from the picture, there are two more threads that may allow for it to fit in a larger opening.
4002698
The hole where the mouthpiece opens and closes to let air escape while drinking does not match up so it doesn't allow you to take big sips.
1855529
The stopper looks nice on the outside, underneath there is a whitish plastic. It did fit over a bottle I opened, it was quite easy to pump. The instruction says to pump until the top stays in, which I did. After a couple of hours it was still in, but the next day when I checked the bottle, it was all out. I pumped it again, and by the nighttime it was popped up again. I do not think it did anything to save the vine, and we had to drink it that night or it would spoil.
It also adds quite bit of height to the bottle, so it might be a problem to find a space for it in the fridge. You would not want to lay down the bottle for sure.
457370
Large ones held a full bottle of wine.  Not one of the ones I used thus far leaked.  Make sure you tighten the cap all the way and wash before use.  I will buy again once I've worn out these ones.