Amazon Kindle Prime Readers & KDP Select Authors discussion

When I published my first novel almost two years ago, I did not join KDP Select, because I was told that if I did, I could not sell my books through other sources, such as Nook.

Can anyone tell me if that is still the case? Or any limitations or catches I should know? Can those of you who are Select authors tell me what you find most valuable about it?

Thanks!