From personal use with XDoclet, it's a great tool, but it's something you have to buy into. If you don't consistently use JavaDoc statements already, odds are you won't use XDoclet consistently either and that is it's one drawback.

If you use it, you have to use it everywhere, and use it consistently. Otherwise your code will break at each build.

Other than that it's a great tool. There are NUMEROUS packages/schemes you can use it for, descriptors, Hibernate mapping, and many, many other routine things. You can also create your own XDoclet extension if you want to do something really special. There are an amazing number of things you can use this beastie for.

You pretty much have to use Ant though to do the automated builds and such (but you should be doing that anyway).