I like the idea behind GOG's DRM-free games, but admittedly the convenience of Steam auto-updating my games for me and keeping everything in one easy to access location has often gotten me to buy games on both Steam and GOG. While I think Steam is the best client at the moment, I've had a lot of complaints about how it works over the years, so I'd be pretty happy to be able to eventually phase it out with something like GOG Galaxy.
Especially considering one of the points they made in their announcement video: I have quite a library with Steam, and all those games are locked to my Steam account. That's going to make migration to another client painful, and will require me to always have Steam installed as long as I want to play those games. It sounds as though things would not be as painful if my game library was with GOG Galaxy.