I can rattle off reasons why I wouldn't use Scala at my hypothetical startup all day, doesn't really matter because that's not the question we're faced with here. I was just answering the question asked above, which was predicated on "heavy users". And the answer is that Scala right this second is awful for heavy users, primarily because of build times. That's the answer, even if you think it will get better and Scala will end up being the best JVM language for big projects.