XMI reality check

One of the issues for the Business Process Editor has been
what format to use to store business process models for
interoperability with UML tools.  I am also encountering
this same issue in other projects.

XMI seems to be the "standard", but I have also heard 
lots of complaints.

This is a general call for feedback on XMI, from people
who have tried it:
* What UML tools have you tried XMI with?
* Have you tried to take the same XMI model and 
  move it from one tool to another?
* What problems did you encounter?
* Were XMI models imported into a UML tool
   really ugly, even if they might have worked
   technically?
* Any other XMI comments...
* What's a better format, if any?  (RDF?)

Thanks a million,
Bob Haugen