Ha. We were required to use Poseidon for UML, but it was such a pain and crashed so often that we ended up making all our UML Diagrams in Dia (which is a nice enough and lightweight program, if slightly clunky; you might want to check it out). Yet instead of being pleased or even just ambivalent about the entire enterprise, our teacher got mad that we used a tool other than the prescribed one. I never really understood why people would get pissy about what tools you used instead of focusing on the actual output.