I was a Picasa user for a while some time ago but decided that Flickr was better for my needs. Back then I used Google+ to upload and share images with other Google+ users. I still do. Google announced the new Google Photos as both a web service and an app at Google I/O. I was sceptical but decided to give it a try on my iPhone.
Google Photos can back up all the images in your iPhone's photo library. Just like Flickr, Google provides a desktop up-loader. There are two backup options that vary by upload size. Original is a full resolution backup of your iPhone images. The storage space used counts against your Google storage quota. I had that turned on initially but quickly realized my error when I got a message that my 15 GB of FREE Google storage was filled. I deleted the images and set my upload size to high quality. That let's Google make decisions about how to compress my images and cut the use of storage. This option provides me with free unlimited storage. The downside is that I have no control over the quality of my images. I am at the mercy of Google's algorithm.
Once you start syncing your images, changes you make to your images will sync as well. Changes you make to your photos, like editing and deleting, will happen on every device that syncs to your Google Photos library including across your desktop and iOS photo libraries. Be careful with the last one. Deleting an image from Google Photos will delete it from your iOS library.
Brian Young wrote an article for Petapixel comparing the compression quality of JPEGmini to that of Google Photos. His results were mixed. Don't depend on Google Photos as a backup option. Uploaded images are limited to 16MP. Use it for its intended purpose. Uploading images for social sharing.
Overall, I think Google and JPEGmini both have impressive algorithms. Google’s beats JPEGmini in compression some of the time, but it looks like there are still some kinks to work out to prevent aberrations and other artifacts.
Flickr gives me 1TB of storage and doesn't limit the size of the images or the quality. I have 49MP images on Flickr. I haven't noticed any compression artifacts after uploading JPG images. One great feature of Flickr is that I can embed images into any web site including sharing to Facebook, Twitter etc. I'm not sure you can do that with Google Photos.
I started this blog post to review the feature of Google Photos and I realized that they were all underwhelming. Meh! All of what Google Photos offers, I can do with Flickr while enjoying better image quality and sharing options.