This seems to be a theme lately where I'm late to the party, taking a long time to get something that is common knowledge to most folks.
Groovy / Grails Support
I was beating my head against the wall using GGTS. For example it was astonishing to me that GGTS didn't understand that I should have a request available to me while working with Controllers. No code completion, I was all on my own. GGTS couldn't help me out at all. Man I hope it's a PEBKAC problem but I couldn't get it working.
So I bit the bullet and started evaluating the newest version of IntelliJ. After meeting some resistance[^1] at work to justify the expense I decided to pay for a license out of my own pocket. That's how good I think it has been for my productivity.
Scala and PlayFramework Support
What I have done in my day job is use the TypeSafe Scala IDE for my Scala programming. Really that wasn't going too bad at all. TypeSafe is dog-fooding it and they're doing a pretty good job.
IntelliJ also has great support for Scala and the PlayFramework. After hours I'm playing with Play, no complaints there.
I haven't fully pulled in our large Scala code-base at work quite yet. We'll see how that goes, not expecting much trouble.
One of the big things to re-map ⌘-R to re-run my last tests. I do that in Eclipse and it works out really good. I'm a very proficient with the keyboard but day to day the function keys are dead to me.
Another big thing that has been helpful is the IdeaVim plugin. It's 95% perfect for me. I'm real quick to get around using it. The last 5% that doesn't work the way I want is really just indenting and outdenting a block of text. It doesn't work the same way VIM does. It's great to be able to keep my brain in VIM mode.
[^1]: It's not quite as bad as it sounds, they paid for an older version of IntelliJ for me previously but I wanted them to pay for the latest version.