I've been using this a bit lately. It's very easy to throw a basic working prototype together and doesn't require that much boilerplate in order to get widgets on a screen, which is great. On the other hand layout management is a bit of a pain and there is apparently no easy way to say, for example, "these three widgets should take up 60%, 25% and 15% of the available horizontal space, respectively". I think that is just a consequence of being immediate mode.