I've been using Wine Shields (Preserva) for several years and can say that they absolutely work. A bottle opened one day and treated with the shield will be quite good the next day. Now, it's important not to expect too much--these devices block most air (oxygen) from getting to the surface of the wine remining in the bottle, but of course the bottle has been opened and exposed to air, and some will get by or diffuse through the edges of the shield. So it's not a Coravin, and it won't preserve precious bottles indefinitely. But it's a great way not to waste a good bottle just because you can't finish it in one sitting!
I can now buy a nice bottle of red wine, have one glass with my dinner, and enjoy the wine for a full week, rather than have it start to turn after two or three days!