That was a very brief flirtation with Expression Media for me. I downloaded, installed it on my system that I just upgraded to 10.6.4 (that has been rock solid for me, so far) and asked it to create a catalog. Within minutes it had crashed my whole system, which I don't think I ever recall happening to that degree. The finder couldn't even recover from a force quit and I had to do a hard restart. It is quite possibly something system specific for me, as I have read of others here using and liking it, but I don't need it desperately enough to venture another try at it right now. Thankfully, as a Phase One owner, I have no quarrel with the price.
We've ran some tests with 10.6.4 with no issues so it could be something specific to your setup.
It works with Leaf .mos files, Phase One IIQ, .TIF and many other image formats inc. .tif etc...
So if anyone here is facing an issue it would be good if we get all the info through support