I agree, I don't see a reason to switch from Groovy. I don't see the Scala or Clojure people switching, either. But even if you're just trying to convince Java developers, Groovy's ecosystem (including Grails) is hard to beat.

And actually, I'm using Groovy++ (http://code.google.com/p/groovypptest/), which addresses the concerns about static typing, inference, and performance, all for the cost of an extra jar. It's really surprising how few people - even Groovy programmers - have heard of this. I think it really ought to be included in the core language.