Ranked in these QuestionsQuestion Ranking
Pro DRM free as a rule
GOG refuses digital rights management on principle.
Pro Seasonal sales
gog features summer and winter sales every year and offer quite a lot of games at lower than retail prices.
Pro Money-back guarantee
If GOG can't help you fix a game, you're free to refund it within a month of purchase.
Pro Money back regional pricing
GOG may charge more in some regions as publishers request so. But in those cases they give the extra cash back as credits for buying another game.
For example, the lowest price for the game is U$20 in US, but if in your region it costs U$30 they will give you U$10 back as credits for your next purchase.
Pro Client is completely optional
You can either download games directly from the site or use their Steam-like client, GOG Galaxy.
Con Limited, often dated, library selection
Con No Linux client (yet)
They say it's coming...and you can still install Linux compatible games directly.
Con No compatible support with (Steam) achievements
Steam has achievements that many appreciate. Unfortunately, those same achievements are not available when you buy the GOG version of the same game.