To answer the more black and white questions first:
- Is there a Java version available?
Yes, but its not yet publically available, you may be able to get a sneak preview if you contact SDL.
- Can it work with Grails?
I don't really have any experience with Grails, but it may be possible to integrate it as they both use Spring MVC. How much additional work this is I have no idea
More difficult to give a definitive answer on, but I will have a go:
- How much benefit is there from DXA if I want to use my own HTML design?
If you have a decent working knowledge of NPM/Grunt, you can use the framework in DXA for building and distributing your HTML design, however this is only of real benefit if you are interested in managing the HTML design from the CMS. Many projects I have worked on prefer to deploy the HTML design as part of the web application directly to the web servers. That said, there are still a lot of goodies in the DXA other than the HTML design - this leads to your first question
- What is there in DXA other than a wrapper around DD4T and integration with Experience Manager?
The answer is quite a lot - check out the list of features in the documentation. In addition to this there is a powerful generic DD4T -> View model mapping framework, which enables you to work with strongly typed views. You can read a bit more about it in the documentation. Note that this is a feature that is also coming in DD4T 2.0.