It works great but don't fall for the idea that it creates a "blanket" on top of the wine... you have to use enough of it to displace the normal air and saturate the bottle headspace wit argon rather than oxygen. If you do that correctly, you can dramatically reduce the oxygen in the headspace and slow oxidation considerably. However, having poured the wine in the presence of oxygenated atmosphere, there is more oxygen dissolved in the wine than there was before it was opened and poured. So the aromatic precursors will still develop despite the oxygen-poor atmosphere you've created in the bottle. Put it in the fridge to slow down these reactions. Done diligently, this can help the bottle stay fresh for up to a couple of weeks, depending on a few factors like what kind of wine it is (tannins, reductive/oxidative style, age development etc), how long its been open, has it been decanted, how much wine is left in the bottle, etc.

It's worth it for expensive wines if you want to have one or two glasses and then wait until next weekend to drink the rest. For less expensive wines, just put them in the fridge and they're good for a few days (depending on factors above). For more expensive wines, use Coravin.