You must make your own decision about if you need beersavers from the folks at Beer Interactive. They promote these silicon caps primarily because they keep your beer fresh and because they can double as bottle charms (you know, like the cute things party guests hang on their wine glass so nobody drinks from it).
If you generally empty a bottle into a glass before drinking then move along to another gift suggestions.
Otherwise consider this very useful review from Mario Rubio, who gave them a thorough test with several different beers. He wrote, “I was impressed with the Beer Saver caps. If you’re the type that has trouble finishing these more common 22-ounce, high gravity barrel-aged beers or simply want to save some of your beer for the next day, Beer Savers area good solution.”