Save webpages to read later, and eliminate cluttering of bookmarks with sites that are merely of a one-time interest. With Pocket, you can read at home, work, on the plane, or during your commute; even without an internet connection.
Pocket is available as a bookmarklet, as a Chrome addon, integrated in Firefox, as several unofficial browser addons, and as a mobile app available for a variety of platforms.
Not reliable. I've had several devices completely stop working with Pocket. Can't save. Instapaper at least has backward compatibility, even if they make the whole app useless by making it impossible to go to the original URL.