Unfortunately, more attractive language to me, Groovy++ didn't get much traction.

Nowadays, Groovy seems to fading as well.

I'm starting out on Grails, do you suggest I shouldn't?

Our requirements are that whatever I choose must be Java-based, and have some nice framework.

Within those constraints, is there a good Grails replacement?

No, not at all. I meant in terms of hype and popularity. Otherwise, it seems quite solid to me, and they (Groovy and Grails) are in active development.

There is another interesting jvm language: http://gosu-lang.org/compare.html

Web framework for gosu: http://ronin-web.org/

But it is even less popular and I'm not sure if it even has an active community.

Gosu is the language developed by Guidewire, a major insurance software provider :) (I work in the insurance sector. I wish my company bought Guidewire).

But yeah, it's much smaller than Grails.