Groovy is getting more and more interest in the Java community.
Its kind of faster to write, especially if you implement detail-rich business logic, which is typical for Domino projects.
Groovy might even be easier to learn than full blown Java and as a language its far more feature rich and more easily extendible than LotusScript (for example collection classes, cool features for xml processing, etc. much more).
I don't want this as a Lotus Script replacer, but as an aditional option.