I've stuck to using GWT generated JS in a hidden webview on iOS, rather than J2ObjC.
I've found the process of compiling code on J2ObjC to be difficult and laborious. It may have improved but on initial investigation I ended up hacking a script to try and generate the correct command line for my project. Not to mention downloading and unpacking each library jar by hand.
GWT on the other hand integrates easily with Maven, pulls in all your dependencies and filters out unused methods.
Additionally, the code can be run anywhere else JS is available, in our case Windows Phone and Xbox One.
GWT was also an idea whose time had not yet come. I'm sure that the success of InBox makes the developers forget some of the blood, sweat and tears it probably took to toolsmith a cross-platform architecture. But they took a conservative approach, sticking to networking, sync, and local persistence. They picked parts of the app that would be using similar-enough APIs on all platforms that the abstractions don't get out of hand, and the learning curve for maintainers isn't too steep.
If I was looking at a big-enough project to justify the toolsmithing, I would be looking at how they did this.
Any opinions from anyone else who tried using it?
I checked StackOverflow and there's only 21 questions with the tag (http://stackoverflow.com/questions/tagged/j2objc) so that put in the nail in the coffin for me. As an aside, that's how I make most of my decisions between frameworks and libraries, I just pick the one the highest amount of SO tags!