If I could do it again, I would assign multiple vlans for server groups i.e. all netware servers in a vlan, all NT boxes in another one, and a few more for the unix boxes. On the user side, I would assign a vlan per workgroupswitch or per wiring closet. In this way you can maximally prune the vlans on the trunks. This is a model that I expect to perform better then what we have now. What most worries me is the background traffic on the server-vlan (our only one) which is 30-50kbit/s during office hours. On the other hand, we have no acute performance problems and the adminstration is easy. Assigning more (server)-vlans will make life more difficult for your administrators.
