A couple of things on my experience in moving my entire photo library (maybe 60,000 pictures) into the iCloud (http://www.apple.com/icloud/photos/). Previously I had a large photo library and started keeping the oldest years on an external SSD (LaCie). The issue with this was that to get to old photos I had to switch iPhotos (now Photos) to it which could be a pain to find old pictures and Faces was per library. This also inhibited me from easily/quickly sharing old pictures. I was also concerned about loss so I had set up a synch system with AWS Glacier.
I decided to make the transition a couple of months ago and go into a single easily accessible storage and access system so I picked the Apple iCloud Photos. Doing that also lets you have a single location for all of your albums instead of multiple sets of albums in different libraries. Transitioning to it took several days of upload (from 2 different MacBook Pros because of the external library) but once that was done it has worked well so far. I also selected the reduced storage option for my iPhone 6 so it keeps all of the pictures in a lower resolution on the phone and the full sized images in the cloud. This has worked well so far but one thing that I do notice is that apps that can select photos take a bit longer to initially load the thumbnails because of the number of thumbnails to deal with. The reason that I had to use 2 MBPs was the iCloud Photos is linked to the device so having one MBP complete an upload and then switch to another library runs the risk of losing all of the previously uploaded pictures. This is because (I think) that switching libraries causes iCloud Photos to start a 30-day timer where it will expire the pictures not in the library that your MBP is using. If you use 2 different devices to upload iCloud will merge all of the pictures into a single library which then can be accessed from all of your Apple devices.
I also had to subscribe to a larger iCloud storage plan to hold all of the images. I feel like the convenience of access and automatic backup offsets the additional yearly cost.
Something else to note is that Google Photos has been in the process of uploading the photos from the phone for quite a while. It is automatically creating memories which is pretty interesting in that it uploads the most recently taken pictures from the phone and then keeps plugging away at the oldest pictures that it had not seen before because they were on the external drive. Because of this I regularly get “memories” of events that I have not seen in quite a while which I like.
So far I am satisfied with my experience with my only real complaint being the additional time that apps take to load thumbnails.