I'm running 9.0M1 Build 10372. I see there's an IDEA "Maia" Build 10666 available as an EAP version. It's not a 9.0M2 release, so I'm not sure if Build 10666 is as stable as 9.0M1.
Should wait for an official 9.0M2 release or is 10666 what will be in M2? Will there even be an M2 release?
Sorry for the confusion.