I had to export a class diagram in XMI format from ArgoUML to Poseidon today. The export/import went through without a hitch; only problem is, the graphics disappeared somewhere along the way. All the model objects were faithfully reproduced in the explorer window on the left, but no boxes or lines. Looks like neither of these tools implement the diagram interchange specification. To be fair, I don't know whether this spec has been adopted by all the tool vendors yet.
The reason for moving to Poseidon is that there doesn't seem to be any way to specify user-defined stereotypes in ArgoUML (or, if there is a way, I couldn't figure it out). Anyway, Poseidon, in addition to having this feature, also feels slicker and faster. The only catch is that even the Community Edition seems to be limited by an evaluation key and trial period.