Initially, I tried Poseidon. I quite like it until I work on the sequence diagrams. I think it has serious problems in drawing Sequence Diagrams. I check with others in this forum who have used it but they said it is OK. Another problem I got with Poseidon it that when you save the diagram as a GIF file, there is this Poseidon watermark. But it can be easily removed if you have Photoshop or other similiar software. Having said that, you should remember it is quite a lot of work removing watermark. (Unless SUN's people doesn't mind). Please try the Sequence Diagrams of Poseidon and let us know your experience.
In the end, I got MagicDraw special evaluation copy for a limited time that allows me to save unlimited number of classes. I think this product is very excellent. The best of all the UML tools I have used which include Rose, Together, VP, Poseidon and so forth. However, the normal evaluation copy for MagicDraw only allows diagrams of up to 10 classes to be saved. This is definitely not enough for SCEA.
Dia is great... I think a good aproach is to do the whole work in some pro software like Rose , taking notes about the stuff considered "wrong" and later use Dia to remake the diagrams following the correct specs...
You have to send them an email and explain your case. In my case, I work for a polytechnic and I was evaluating the software to be used for teaching. I am not pleased with the expensive ones like Rose and Together. So they give me a 30days unlimited evaluation key.