I'm working on an RPG-ish game that will need a fairly robust GUI. Do not tell me to write my own GUI.
I've narrowed my options down to libRocket and GWEN and now I can't decide. How many of you have tried either of these engines, and what was your experience with them?

I'll share my experience with libRocket, which I have both used and have contributed Lua bindings to.

If you have experience with HTML4/CSS, then you should use Rocket just because it is something you are familiar with. However, it also comes with the inflexibility of HTML and CSS, and things like animations will have to have special element types that you make yourself. However, almost anything is possible without too much work. It does come out of the box with its own look and CSS page for its 'Invaders' demo which gives you a good starting point to branch off from.

The thing that sets Rocket apart is the in-game debug window where you can inspect elements and see its parents,children,style sheet, line numbers, files, templates, classes, and the like. It is something that I haven't seen in other libraries, and I have found it incredibly useful.

I could go on, and if you want to know about a certain aspect then I can answer your question. But to sum it up, it is a mature and easily extendable library that does what it sets out to do, and does it fairly well. Just to be clear, it is very far from perfect; I only pointed out the good in this post. However, your issues will more than likely stem from your misunderstanding rather than a bug in the library, which is something that I take comfort in.