With Scala IDE I've only had painful experiences, plus I hate Eclipse with a passion - but folks keep telling me that it got better. Well, I don't really care as long as IntelliJ IDEA exists.
They are also extremely fast and responsive in fixing bugs of any kind. I personally couldn't be happier and other communities can only dream about such IDE support. Of course, if you compare IDEA's Java support with the Scala plugin, it's worse, but IDEA works better for Scala than Eclipse does for Java, like seriously ;-)
I think Scala is a good language, but maybe it has too many features for me. Xtend did not leave me a bad impression anyway :)
Because Xtend is statically typed, the Eclipse Xtend editor has quite an excellent autocomplete. For your cursors's location it can propose you a list of all valid keyword, methods names, filed names, local variable names, Class names, etc.
And by excellent I mean: Autocompletion will not forget things and it won't show invalid proposals. No guessing!
For Groovy this just isn't possible because it is mostly unknown to the editor which methods and fields an object has. It's only known at runtime.
It's true in general, but in most specific cases it is possible to statically analyse dynamic languages to the point of providing meaningful auto-completion. For example in Python-land there is Jedi (http://jedi.jedidjah.ch/en/latest/), and also commercial Komodo IDE and PyCharm. This is done through static analysis, so that no code is ever run for auto-completion.
So, while it's impossible in general case, it's perfectly possible to create useful auto-completion for dynamic languages. There are such tools for Python, JS and probably others as well.