GOG Galaxy seems nice, but I hope the final version will be polished much better.
Its features are limited at the moment. They need to sort out importing an already installed GOG game, because it's messy at the moment. I can only import a game via "Import folder" option (which seems to return my copy of the game to a vanilla state, btw) and telling GGalaxy to use a specific folder as a root of all the games folders simply doesn't work. It doesn't recognize that any games are installed in that folder.
Another thing is localization. If I want to install a Polish version of the game via GOG Galaxy, I need to install the English version first, then go to this game's config and select "Polish", which prompts the update of my game files with Polish ones. This might results in an additional download (depending on the game, I had them as big as 2GB) which is simply pointless, you basically need to download these files twice, once for the English version, then for the other one.
Finally, it needs to have a way of adding friends via their nicknames. Adding friends by clicking on their profile icons on the forum is nice, but it definitely should not be the only option available.