I am still using iPhoto (though playing around in Photos for Mac) due to one reason, really, and that is that in iPhoto I can "round trip" an image into Photoshop or some other external photo editor, and the saved results go right back to iPhoto. Photos doesn't let you do this (yet), so it's less convenient for me to work in.
But other than that, there's really a LOT less difference between iPhoto and Photos than people seem to believe. The more I work with Photos, the more I am struck by how this is really just a "bringing the app up to code" sort of thing, to borrow a construction-industry phrase. Photos is waaay faster than iPhoto, and nearly everything that was in iPhoto is in Photos, its just that the furniture got moved around.
I recently taught a class on Photos, even though I don't use it full-time, and what I discovered among the audience was that -- again and again -- what they were really made about was either the impression that iPhoto had been deleted, leaving them with no choice (which as incorrect, but could have been handled better by Apple) or that there was *one particular thing* they couldn't figure out how to do now that they used to do in iPhoto.
Once I ran through how things were organised in Photos, and answered that one issue they were having, most everyone's attitudes towards the program changed completely, and they were much more ready to delve into it and learn it.
I think I'll probably wait until the first big update to Photos before switching over completely, but that's likely coming this fall so I'm not especially worried about it, since I still have iPhoto. I have the luxury of playing around with and learning Photos better without it being "mandatory" yet, which I find is the best way to handle these sorts of product transitions.