I've learned mean just to be fast. Lots of documentation and examples, I also like that you can setup continuous integration quite easily (compared to maven java in my experience)
Grails, not because it's better then anything else out there but because Groovy is similar in syntax to JAVA and I was able to get productive quickly.
Groovy hasn't kept up with Java's syntax extensions in Java 8. You'd be better off using Java 8 than Groovy, if only because Oracle's committed to maintaining Java whereas Groovy was ditched by its only financial sponsor earlier this year and all the developers working on it were retrenched. The only activity at least one of them has time for is planting blatantly promotional questions and answers on Hacker News.