Groovy support is available "out of the box" while Scala support can be added by downloading a plug-in. I have been reading about both languages and I even started writing little scripts at work in Groovy (utilities to extract data from text files and such). However I must say that the lack of a good IDE was quite a hindrance. As a Java programmer I expect a lot from my IDE being used to programs like Eclipse and Netbeans. Some of the productivity gains from those new languages are not so impressive when one is used to working with a good IDE. Take for example the Groovy (or Scala) "def" keyword. This is often presented as one of the advantages of Groovy over Java. You can replace:
StringBuilder myString = new StringBuilder();
def myString = new StringBuilder()
The problem is that in Eclipse for example when I enter the first expression I type the following:
myString = new StringBuilder();
Then I just press
Another example is the Groovy @Delegate annotation that generates delegate methods for a given member. Again in Eclipse I just right click the member and select "Generate delegate methods" from the Source menu.
Of course, in this category both Groovy and Scala offer much more gains then what an IDE like Eclipse can offer. However, those gains have to be weighted against the loss of other IDE functionality. As a Java programmer and Eclipse user I expect a good browser for my language. A browser is an essential part of a good OOP environment. This is so true that Smalltalk development kits have always included a browser. It is somewhat painful to apply good OOP principles if you don't have a browser (good OO programming tends to result in more numerous small classes).
The other must of course is code completion. The large number of core classes and API makes this absolutely essential.
I think a good open source environment like IntelliJ will contribute to the adoption of both Groovy and Scala.