Recs.
Updated
Specs
Pros
Pro Unlimited storage for small & mid-sized images
Google offers unlimited storage for any images under 16MP (~4.6MB). This is large enough for just about any 'regular' image (most phones and even dedicated cameras capture pictures less than 16MP). If pictures are over 16MP, then they can still be added, however they will count towards your Google Drive storage (5GB is free, but this storage is the total across all of Google's services including Docs, Sheets, Gmail, Google Photos and more).
Pro The built-in editor works pretty well
If you decide to edit a picture from within Google Photos, you have the ability to adjust the light, color, pop, and vingette through a slider. You can also use filters (also applied with a slider bar for just the right amount of filter) and do regular edits such as cropping, rotating, and zooming.
The editor also keeps track of the original image, and it's easy to revert back at any time. Even when you are finished editing, the edit will be created as a new image - keeping the original in tact in case you made a mistake or want to made another edit.
Pro Smart shared libraries
Google Photos allows inviting other users to shared libraries. It has a feature that allows auto-adding all photos and videos or only of specific people to the shared library which is really useful for sharing with families or a close group of friends. It also allows the invitee to save those photos straight to their device automatically as if they took it.
Cons
Con Low quality copies
Google compresses all photos (to JPEG, medium quality) AND caps the max resolution at 16 MP (without paying for a storage plan), which rules out RAW uploads for any of the current name-brand DSLR cameras and up to 8 out of 10 most highly-rated point-and-shoots (DSLRs range from 20 to 50 MP right now, mid-2017; per PC Magazine's 2017 top P&S roundup, Canon and Panasonic sell 12.8MP cameras but the rest are 16-24MP; unknown if the camera maker's approximation of 16MP meets or exceeds Google's limit).
Con No Lightroom integration and no API
Google only allows uploads via its desktop and mobile clients, and has stated it has no plans to release an open API for Google Photos. This means anyone wanting to share from Lightroom or any other major photo catalog app has to first export all their files to disk, wait for the export to complete, then upload to Google Photos. This extra step is cumbersome compared to sites which offer first-party upload plugins and an open API for any other developer to build off.
Con Not a lot of free storage space for high-res content
While there's a free unlimited storage plan, it's limited to 16 megapixel images and 1080p videos. Uploading full-resolution content, if you opt out of free unlimited storage plan, is limited to 15GB that comes with a Google account which is shared across all Google services. Increasing that storage space costs about $10/mo per 1TB of storage.