I'm currently in the process of uploading 12 years of photos to Google's cloud.
The results so far can be described as magical. The service organised my photos by people, location and even subjects (such as landscapes, fireworks, beaches, etc...).
The hourly surprise of automatically generated stories, collages and Gifs makes me return to the app more than once during the day, a wonderful example of how User experience design should be done.
In this article, Steven Levy interviews Google's Bradley Horowitz about Google Photos. The article includes some interesting details on how the technology behind it could be applied in the future.
Right now I'm using Google Photos as a powerful backup solution for iCloud Photo Library. Who know if in the near future I will stop using Apple's Photos service.