Addiction is the commonly-named devil. In today's world, I think it's the only one ever given. It's a good descriptor as it is frequently used by gamers to describe videogames (eg, current-gen MOBA addiction) while being vague enough to allow one to envision it into being as threatening as an unhealthy love of chocolate or as benign as daily meth use. It's also a great way to demonize something without bothering to try and understand what drives the addiction to begin with.
The Benefits: Due to having a third of the population playing the Game, it will be a primary force in the story's culture, especially if that third was isolated in a specific area (say, the developed world). As the Game is Culture, many will play it just to be part of the in crowd. The Game's influence in Economy will be large, and could be similar to how Entropia Universe combines real money with virtual property, Second Life focuses on social groups and custom content, or be based on making transferable wealth through inter-game cryptocurrencies. Likewise checking the current top PC games shows that games focusing on quick slices of action (or rounds), creativity and world-building seem the most popular, especially when mechanics are fairly simple to start. However, games like Eve Online show that adding optional depth can lead to die-hard fans. All of this is based on the merits and technology behind the game, little to do with any dependency. Instead is fillsit fulfills the needs of the player to create, be noticed, hone their technique, and to encounter novel experiences. Basically fits with the Bartle Taxonomy of Player Types.