A custom ROM is a developer's set of improvements to stock experience. So it can make the phone better in many ways, UX improvements, speed tweak, new features, etc. Whatever the developer wants to add in to the ROM. You gotta find one you like.
I second the advice that you should wait. I've only see one custom ROM out for the GNex LTE version and it's good but it's not much different than stock. Just make sure you do your research...since these things are created by developers they do not go through all the same Q/A that stock ROMs do so you are likely at some point to run into issues.