Google Pixel and the full resolution backup
So, didn't Google say "high quality" backups unlimited for free? Yes, they did. But HQ meant to be you can store photos up to 16 megapixels, but NOT in the original quality. They did some magical compressing on all photos regardless if higher than 16 megapixels or not, after uploading to Google Photos. While it was indeed high quality, if you mess around with that photo more, you quickly found the problem: Compression artifacts and color issues. And the most haunting thing for a backup was, it was NOT the original in the best possible quality. If you lose your offline file (you do backups on your computer, right? RIGHT?), you lose your original file and therefore quality. Normally HQ is good enough.
With the Pixel and Pixel XL you have unlimited space for your photos in original, unaltered quality. I double checked and compared the file transferred form my Pixel to the computer and the uploaded backup file from Google Photos via downloads. It is exactly the same image and file size, byte for byte.
This unlimited, unaltered original backup is tied to photos that are uploaded from the Pixel phones. Even if you have such a phone, photos uploaded from other devices or your computer will count towards your storage space or being compressed to "high quality" format like before.