I've used Qt on a couple of projects I did in c++ on several platforms over a period of seven years. I think it works pretty well and definitely was quicker for me to develop a decent GUI app on the Mac than plodding through a language I didn't know (Objective-C) at the time.
I think the signal/slot mechanism is a bit funky but isn't horrible. Once you're use it for a bit, it's not a show stopper. The connection stuff is easy to bungle up (or at least it was) and it's always good to check the return on those because your app will go merrily on its way and not tell you that it didn't work.
I've never used jambi.
I have been using Qt for several years now for commercial development and have been very happy with it.
One of the nice things with Qt is that it provides a large set of libraries as well as the GUI stuff (eg XML parsing, threads, networking), all in a consistent style and all multi-platform. This means we rarely need to use other libraries, though we do use boost for some things.
Another very important factor for us was internationalization. In a previous, MFC based application we had to maintain 2 localized versions, for the two languages we support. In our Qt based app we just have the one version.
One thing that has been very valuable is the access to the source, although e this is certainly not unique to Qt. On several occasions the ability to check the Qt source has explained some strange behaviour or given a clue how to achieve something.
We have found a few bugs in Qt, some of which have been fixed after reporting to Trolltech. In other cases they have suggested a work around. These have all been fairly obscure and not had a major impact on our development.
One of the main downsides to Qt would be the lack of 3rd party libraries for use in commercial applications. However, Qt is fairly complete so for us it has not been a big problem, though that will depend on which type of application you are developing.
I have not used Jambi either.