Mighty No. 9 has a new release date: February 9, 2016 in the Americas, and February 12, 2016 everywhere else. That goes for physical and digital versions on Xbox One, PS4, and Wii U, and digital only on Xbox 360, PS3, PC, and Mac. The 3DS and PS Vita ports will come "at a later date."
Prior to the delay, Mighty No. 9 was targeting September 2015, but "bugs and issues pertaining to the online features" dashed those plans. It's been a messy, drawn-out path to launch.
Folks who backed the game can now play the exclusive demo (which was also recently hit with a delay, because these guys just can't catch a break, can they?). You can get it via Humble Bundle.