Grails 2.x or Grails 3 ? Many sites using version 2.x haven't bothered upgrading to version 3 in the two years since it was released, nor are there many version 3 plugins converted. And very few new Grails projects are started.
We use Grails for a lot of internal applications at our company, all 2.x (2.3.x through 2.5.x).
I can't really comment on Grails 3, funnily because the applications were written before 3.x came out and we haven't bothered upgrading them yet... :) I've only taken a brief look at what's involved in upgrading them, but other stuff has always been higher priority.
It's probably worth being familiar with both versions, because Grails had plenty of adoption before 3.x came out and 2.x is still being maintained.
