Ryuubei: try reading the forum. But from my own experience:
Cant add DLC
Cant update
Crashes after 15 minutes (confirmed this is Galaxy related as the game launches and runs fine on its own)
Claiming the game isnt available yet
Those are just with the game installed, trying to install via galaxy has a number of other problems. As it is right now GOG shouldn't be pushing it onto people. It isn't full featured or fully working (counter to their claim)
It 'could' be worth using when it's actually complete but for now not worth the hassel
For what it's worth, I haven't experienced any of these issues. I installed GOG Galaxy specifically to install Witcher 3 and it's been smooth sailing for me so far. YMMV, but I'm happy to have the easy installation and auto-updates.