This is the favourite UML tool in our office. Sdedit (http://sdedit.sourceforge.net/) is a nice addition for sequence diagrams.
I've gone through a smaller but similar journey through build tools and appreciate that you actually took the time to write down and share your experiences. I find the GBS (autotools) to be very frustrating and have ended up writing my own shell scripts in the past because it was the easiest option. I will definately be trying QMake, CMake and iffe after reading this.
However, I do disagree with your conclusions on some of the tools. Personally, I am thinking going back to make. It took me 10 minutes to learn, uses skills I already have in the shell and, for me, it works in a simple and consistent way.
I moved to using Scons a few months ago and I feel it suffers greatly from being based on a general purpose language. The code does not look like a build description, its too slow on any CPU under 1GHz and I do not find it consistant. For instance, having a 'make test' sort of target that runs unittests took me ages to figure out. With 'make' its simply the same as any other target.
Humble 'make' never wasted so much of my time as the better improved Scons and I fear that this is a problem with a lot of the newer tools. They try so hard to be powerful (possibly because they are written by people that find builds interesting rather than a necessary chore when coding) that they loose the simple elegance of 'make'.