The only area where I have ongoing issues with my IDE (Scala IDE) is with the play template plugin, can really wreak havoc in both template and controller layers with spurious errors that break mouse-hover type inference click-through-to-type functionality. Even here there's been a ton of improvement, 2 years ago in the template layer there was no semantic highlighting, no indication of _valid_ errors, and stack trace error lines pointed to lines in the generated template, not the .scala.html source file, fun times o_O
Recently switched to XML literals in place of Play's @ templates, not surprisingly the above issue has disappeared, and am now left with a fully functional IDE, code completion, refactoring, and the all the rest, pretty thrilled about the IDE story in Scala to be honest.