I migrated from working pretty exclusively with Groovy/Grails to Scala/Lift over the course of a year or so, and have no regrets at all.<p>One of the interesting things I found is that it's remarkably easy to port plain Groovy code to Scala with only minor changes.  Once you get over the (somewhat steep) learning curve of the Scala type system I've personally found that I'm about as productive as with Groovy, but end up seeing far fewer runtime errors.<p>It's not that easy to move existing Grails applications to Lift, however, since the frameworks are so fundamentally different.  I've found the best bang for the buck is in moving as much Groovy code as possible to libraries outside of Grails, and subsequently rewriting them in Scala as needed.