Recs.
Updated
Specs
Pros
Pro Steam sales
Steam has daily and weekend sales as well as publisher sales, though their biggest sales are their winter and summer sales. Steam is known for their deep discounts (discounts on games range from 10% to 95%) on games and has pushed many other stores to do the same seeing Amazon and other storefronts match their pricings when sales go up as well as hold their own seasonal sales.
Cons
Con Buggy client on Linux with little customer support
Linux support is generally overlooked. It is recommended to disable in-game overlay and to use a real browser to interact with steampowered's website. Basic library access works usually without a hitch.
Also do not expect real tech support on Linux. Forum questions of bugs often go unattended and unanswered.
Con Billing address required to purchase games
Which is good because you pay your taxes.
Con Requires to log into Steam every 2 weeks
Steam forces you to periodically log into Steam Store every two weeks, or your games will stop working. They will force you to keep adding the API number, which is a pain. There is an Offline Mode, but you still have to log in at least once every two weeks.
Con Little support for older titles
Many older titles (10-15 years or more) are no longer supported and thus require much manual tinkering to run properly on windows 10, requiring digging though forums and trying many things. Compare that to GoG where these titles often run out-of-the-box.