I guess not available to every user, but I bought a new MacBook Air to replace my 2012 MacBook Pro/El Capitan this week. I already have an iMac/El Capitan, and I have remained using iPhoto on that. I had done the same for the MacBook, though on trips abroad, I have trialled using Photos for the pictures taken at the time, later importing them to iPhoto on my iMac. It's okay for casual use, but I have 45,000 pictures in my iPhoto library and the collection - including scanned documents, spans 120 years. While I have Aperture, and Lightroom, for the vast majority of the time iPhoto is perfect for my needs.
I have bought Intensify and Tonality in an attempt to make Photos more flexible, but the advantage with iPhoto was that it came as the whole deal with the OS and was one of the reasons I originally switched to Apple.
My intention with the new MacBook Air was to use only Photos from the start to see whether it is any more usable - for me - than it has so far appeared.
That lasted all of two hours. I then downloaded iPhoto from the App Store (I had it previously, which is how I could do that), and will continue using it.
It's all down to personal preferences and needs.