Scala provides two things: an expressive type system, and access to the JVM. The JVM is fast, and has a ton of libraries. If you write web apps that fit comfortably in one or two boxes you'll likely never need the speed. If you've never used a language with a modern type system it's well worth learning one, though Scala is not the simplest to pick up.
If you write web apps that fit comfortably in
one or two boxes you'll likely never need the speed.
This is hand-waving the fact that the JVM really is good only for server-side applications, and nothing more; mostly because bindings to native APIs are shunned by Java developers and because Java SE may be fast, but it starts slow and it consumes tons of RAM -- 16 years after Java was launched, 16 years of Moore's law, and Java apps still feel like crap.
It is kind of ironic that if you want to write well-behaved OS X, Gnome, KDE or Windows apps without dealing with Obj-C or C/C++, you're better off building on top of Ruby/MacRuby, Python or Mono/C#.
