How much of this is (b)?
That said- it's not a superior language, just a different paradigm. With Scala you get very strong typing, functional constructs (so incredibly powerful), matching, options, very clean closure syntax, etc.. You'll eventually fall in-love with _, too.
I haven't used scala.js yet, but here are some downsides to Scala proper: * slow the compile * tooling isn't great (this true of anything jvm to some degree though)
Well, you have plenty of very decent tools to inspect a running JVM state, and that's priceless. Eclipse is alright if you manage to set it up once with the tools you need and don't fiddle with the setup afterwards. Although slow, it packs a lot more features out of the box than Visual Studio. The debugger is pretty good too (including remote debugging).
If you compare it to state-of-the-art tooling for more fringe languages like Python or Haskell, it's lightyears better, really.
If you compare Scala.js to similar languages (CoffeeScript, TypeScript, ClojureScript to name a few), it definitely helps to have the same language both on server and client, especially if logic must be replicated on both sides. So in that sense, there is some (a) too.
But that is only my opinion. Any developer or team can make up her own mind on the matter.