Image previews get lost in all IDML use-cases.
When you open up an IDML generated by CS6 in CS5.5, any feature that is CS6-only will be lost.
The text engine changes from version to version, so you can't rely on the rag to be identical when you round-trip through IDML.
Additionally, I've seen a variety of hard-to-track imperfections in interpretation of IDML, in CS6 specifically.
You need to get a CS6 upgrade license, yesterday. Your workflow is dangerous.
What can occur (it occured to me): an applied paragraph style was changed to a different one.
Basing a roundtrip workflow through IDML is really a bad idea…
Joel is right calling it outright "dangerous".
Ah, just another one, I have to admit, a very obscure one:
Whole tables will be lost, if they exceed 200 columns (not so with CS4).
It's unlikely you ever will work with those, but… ;-)
Dn't do that. IDML is not a workflow to going back and foreward between versions in a workflow.
You will loose new features, but also bugs are changing the files, like loosing numbers and other unwanted losses.
Your workflow is really dabngerous. You have to use the very same version and build as your client does.