Mandrake,
I haven't made many miles on a Zieleman yet, but that they ride beautifully is what I hear all the time from people who did ride them. Those people not only rode Zielemans. They also raced with frames from other very well known established names. I did make many miles with my Jan de Reus. The Reus suits my preferences very well and so far I consider it the best riding bike given the circumstances overhere.
On the build quality of the Zieleman I can say that they are all one offs. Zieleman like the white one from the previous page and my blue/yellow one show a lot dedication to doing everything very tidy. They even look flamboyant, with all the chrome. I have also seen Zielemans -my golden one is a good example- that seem to be built with a more pragmatic attitude. Ko spent a lot of time on the rear drop outs and apparently when everything had put together there was no time left to do it in a very tidy manner. What all Zielemans do share is a sincere vision by the builder on what works best for the rider to make him perform as good as possible in competion. That is what the frames are build for. Remember that for Dutch a bicycle is a quite utilitarian piece of equipment and that is reflected in the Zieleman quite well.