I'm not even sure what Roblox is exactly. From the few screenshots I've seen, I have a gut feeling it's not for me.
Graphics aren't that important to me, but Roblox's graphics really put me off. So no, I've never played it.
Don't know a thing about it other than what some articles are saying that it's full of potentially dangerous child-adult interactions. And that no one's doing shit to fight it.