Apple seeds OS X Yosemite 10.10.3 build 14D72i to developers on Thursday, February 5, 2015 along with Photos App.
Photos App for OS X Yosemite looks similar to Photos App for iOS 8. It performs really well on 2014 Core i7 MacBook Pro.
Apple says that Photos App is coming Spring 2015, which ends on June 20; after WWDC.