They do leave a needle sized hole in the foil and cork (the cork self seals) but they are great for checking on the state of a bottle especially before a dinner where you can taste the wine without needing to open the bottle. If it isn't what you wanted you can still keep the bottle unopened indefinitely. I use it to pour a glass then set it aside and finish it another time. I have tested this a number of times (unscientifically) and haven't seen any fall off in the wine.
I've certainly heard a lot about them, not least of which was about the
fear that they could be used for fraudulent purposes. I wouldn't mind
alt.food.wine FAQ: http://winefaq.cwdjr.net