I'll be honest, I'm personally not a fan of Scala (I prefer Clojure for my JVM goodness), but Scala does give a lot of niceties IF AND ONLY IF you're willing to learn how to program the more pure functional side of things.
A lot of Scala converts fall into the trap of "writing Java in Scala", and if you're doing that I'd argue it's not really worth it...at that point, you can really just write Java; however, if you're willing to learn functional principles (including some basic category theory), it has an incredibly powerful Hindley Milner system under the hood.
Spark is a really useful tool, and the Java bindings work well enough, but it clearly is a "Scala-first" framework. For that alone, I'd argue getting proficient with Scala is probably worth it.
My biggest complaint with Scala is that, because it's very much not opinionated, you end up getting paradigm-clashes with larger teams of people. This will happen to some extent with most languages, but I find it especially egregious with Scala. If you decide to use Scala on a team of multiple people, I cannot overstate the need to codified standards of what is allowed and not allowed.