I've been a longtime user of iPhoto Library Manager, so I waited until PowerPhotos was available before migrating my photo libraries to the new Photos app. PowerPhotos is just like iPLM, and that's a good thing. It's easy to use, it's fast, and it's supported by a dev with a great track record. I particularly like that I can browse all my Photos libraries at once, and I don't have to open Photos if I simply want to use an image in a document or send it to someone. and if you already have a iPLM 4 licence, PowerPhotos is free, which is a really nice thing for the dev to do for his existing customers.
PowerPhotos doesn't yet have all the features of iPLM, but its only version 1.0 and the dev says more features are coming. In addition to the copying, merging, and rebuilding features of iPLM 4, I hope that something like the iPod Folders feature from iPLM 3 makes it into PowerPhotos, as I used it all the time with iPhoto, and I'm going to be lost without it!