iPhoto on Yosemite will not open
If you've "purchased" iPhoto before, trash the old version and download the current version (9.6.1) from your Purchases tab in the App Store.
Edit: updating won't work any more (Apple wants you to use Photos).

PS.  I have Photos, Aperture and iPhoto 9.6.1, the latter I can download any time from the 'Purchases' page in (UK) App Store and launch/use it.