I gave it a shot and it worked partially, but not like it was supposed to.
I had 1.7 GB of photos on my iPhone (around 1000 pictures). After I turned on iCloud Photo Library, it started uploading them, which worked fine. I selected the optimize phone storage option. Then weird things started happening. First, the option to use iCloud Photo Library completely disappeared from my settings. Then the amount of storage taken up by photos increased to about 2.2 GB instead of going down like it was supposed to. After the photos finished uploading, I checked my iCloud storage and it said that there were 1.7 GB of photos stored in my iCloud Photo Library.
Then I checked my iCloud backup. I had done a backup prior to starting iCloud Photos since it's listed as beta and I didn't want to lose anything. It was 2.4 GB when I backed it up. When I checked it though, it had changed saying that my last backup was only 700 MB (as if the photos had disappeared from it). Also, it said the next backup would now be 4.4 GB as the camera roll (which ironically no longer exists but it still uses that name lol) was now 3.3 GB of files. Eventually, I just turned it all off (the settings option did show back up eventually) and then deleted the old backup and did a new one and everything was back to normal but it was really weird.