Whether or not it's legal doesn't matter to me really, I doubt it would be any easier for a state here in the US to leave the union.
I don't see a problem with people wanting to be governed more locally instead of from a city that's much further away. Especially if they have a language and culture that's different from the rest of the country. I'm sure most Catalonians don't hate Spanish people and I'm pretty sure they could get along fine as separate nations.
I'm not sure how it'll go economically though, not sure if the EU will let them in but from what I've seen a bit is that Catalonia is a pretty wealthy part of Spain at the moment.
So ya obviously just from my view here in the US I have no problem with Catalonia leaving Spain. This wouldn't really be a big deal if Spain hadn't tried to stop the voting in the first place.