Note: Amazingly, I wrote this review way back in 2007. I just tried to use Poseidon UML again in 2010, and I wouldn't change a word of it. (Okay, I'll change one thing: I now use UMLet instead of MagicDraw UML.)
Poseidon UML is the UML tool that continues to disappoint me. I keep rooting for it, because I remember the early days when Jason Robbins was creating Argo UML, and I thought he had some really good ideas on perspectives and critics. But every time I try to use Poseidon, it remains too buggy. The last straw with the latest version is when it locked up on me while I was teaching a UML class. I have a copy of MagicDraw UML and switched back to it then, and haven't looked back.
I'm also disappointed because I thought that after all this time the Gentleware people would have made some significant improvements to the Poseidon UML user interface. It still continues to feel clunky. Just try to draw a simple Use Case diagram, and connect lines between the actors and Use Case bubbles, and instead of straight lines between the bubbles and actors, you'll see the lines jump all over the place. That's a very 1990s approach.
I see that they have an Eclipse plug-in now, but I think I'll wait until 2008 to take a look at that. Until then I'll keep using my old copy of MagicDraw UML.
Update: For a review of UML tools, please see my Free UML tools review.