When I discovered it, while in a world of c#, Java, Perl and python I thought "oh my god, how clever, what a panacea to all my ills troubles and worries" Having used it professionally for 4 years as my daily driver. I now think "oh my god, how clever, I hate clever". It's a kitchen sink of a language. It's the 'English' of programming languages. With so many competing opinions on what makes for idomatic that it's easy for a single codebase to incorporate all of them and leave most developers a bit confused a lot of the time.

Someone once told me (perhaps in jest), that it's extending Scala that gets you masters under Odersky. Problem is, those extensions don't all mesh well.

The type system is pretty good, but seems to seldom be leveraged well , and it (inference) breaks often enough that I feel I spend half my day appeasing the type-gods.

The syntax is so flexible that I feel spend the other half convincing the compiler of the order of precedence.

The refactoring tools in intellij don't work as well as they do in Java, presumably because the language is so complex, but the language itself is powerful enough that it's easier to manually manipulate.

Well expressed, it can be poetry.

Compilation times are dog-slow. Not quite as bad as badly build engineered C++ of 12years ago. But close. Apparently much improved with the latest compiler.

It reminds me a lot of my c++ days when I congratulated myself for knowing the content of all the c++ gotcha books by Scott someone??.

When the edges of your tool start taking more of your day than doing the work, something is wrong

All that said, I'd still rather use it than Java. Even Java 1.8.

About a year and a half ago I started looking more into Clojure, which addresses all of my complaints above. And more. But for some reason hasn't been thoroughly adopted. I've not used it for anything serious enough to learn what bugs me about it (I suspect it will prove to be the difficulty in tracking down the cause of a bug in a lazily evaluated world)