I'm about to start a project (which isn't a game) written for linux in C/C++ and need to decide what to use for the GUI. I know of QT, motif, GIMP, etc. and am doing independent research on my own, but I'm also looking for some testomonials from people who have used them before. It needs to be very easy to use (the GUI, not the API), and have support for as many features as possible.

One thing for sure, QT has most features of any framework I came across. Furthermore it has a good gui builder and IMHO the OO nature makes it quite easy to develop for. On the other hand GTK (with Glade) is also quite easy, C based, but not as packed with features.
I prefer to use QT with c++ and GTK with python, but thats just a personal preference.
I also remember having used FLTK for some OpenGL project and it was a pleasant experience - not many features, but does what it should - drawing windows/buttons and being extremely low weight.

I tend to use either wxWidgets for GUIs with lots of features, or FLTK for smaller stuff. Both are C++ based and have GUI designers available: FLTK has Fluid which is part of the toolkit, for wxWidgets I've had some experience with wxGlade and that's the one I would recommend.

I've been pretty impressed by both. I'm guessing you would probably want to look at wxWidgets if your GUI is going to have a lot of features. The other options aren't bad, with the exception of motif: it has to be the ugliest widget set I've seen.

edit:
I also know of people who like Fox. I'm not really making a recommendation one way or another. I just thought I'd mention it in case you weren't aware of it.

I've found gtkmm to be very convenient. Qt of course is good too if you don't mind the license and the language extensions.

I recently started a linux GUI project so I had a little look at the various APIs. In the end I went for QT since the documentation was very good and the API seemed simpler than say GTK. Actually, I was incredibly suprised how easy QT is to pick up. Also, I have heard that QT on windows is more stable than GTK.

On the other hand, GTK is free for commercial software whereas QT isn't and for all I know GTK could be just as easy to pick up.

My project is relatively small and I was more concerned with development speed than performance. I have no idea as to QTs relative speed or overall functionailty compared with other GUI APIs.