It's true that Sonic Runners has been out for a few months, but this was only in Japan and Canada for testing purposes. By next week, this soft launch period will conclude, as SEGA has confirmed today that Sonic Runners will officially release worldwide (along with an update) on June 25.
Sonic Runners features familiar pals Tails and Knuckles as playable runners alongside Sonic himself, in addition to cameos of other characters from the Sonic universe. If you've played Sonic Lost World, you'll notice an air of familiarity as the game uses some of the assets and environment backdrops from that game. In fact, references to other Sonic games can be found throughout, including a small tribute to Sonic and the Black Knight with the rather obnoxious Caliburn as a possible travel companion.
Recently, SEGA celebrated 100 million downloads with Sonic Dash. When you consider that major milestone and now with Sonic Runners also entering the space, it's clear that Sonic has found a comfortable place on mobile platforms, giving credence to SEGA's mobile push as being a worthwhile decision.