I upgraded to Yosemite on all of my computers. iPhoto had several themes that were dropped in Photos, more specifically 'Places' which worked great for me as I travel a lot. I spent a lot of time building attractive travel slide shows using the 'Places' theme. All that work got lost by moving to Yosemite/Photos!!! In addition, the older versions of iPhoto won't run under Yosemite, and if I try to upgrade to the last iPhoto version (which runs under Yosemite), the upgrade is not available anymore.
Personally, I liked to work with iPhoto and Aperture, which is not supported either. From my perspective, Photos is a step backward!!
It would be great if Apple would at least provide the last version of iPhoto in the App Store, which would leave us with a choice!!! Or warn iPhoto users.