Yes, several similar threads exist, but we're now in 2011, and much has changed.
Grails 1.3.6 has improved hugely compared to v1.3 when I initially tried to learn the framework (and gave up to slow compile times and other groan inducing events).
Having spent a couple months with the latest release, I am impressed, protyping an app is an absolute breeze (GORM is great!). In development mode one no longer needs to restart, barring changes to domain classes. Groovy.lang is fantastic (bare in mind, this is compared to my day job life in PHP).
Now, on the flip side there is Ruby/Rails, which I have little experience with beyond perusing Ruby docs and exploring Active Record (to compare to GORM). Coming from PHP/Jquery, groovy syntax is cake, ruby not so much, although accessible.
Ruby/Rails is all the rage, while Groovy/Grails seems to be picking up speed in its own right.
I'd like to hear what both camps have to say (flame inducing lang war welcome) re: pros/cons of both langs/frameworks now in 2011. When choosing a framework it's important to know what you're getting into, so have at it, newbies will benefit, and experts can vent ;--)
Rails and Grails are both excellent frameworks with their current releases. You really can't go wrong with either. Here are some things I find interesting about them though:
Rails
Grails
My perspective
My Recommendation