I've seen it - some interesting possibilities and some limitations. I'm not an expert by any means, but from what I've seen, here's a few thoughts:
- you need an Analyzer license
- with you can drag n drop custom object fields to create the report and, to some extent, related mapped data (e.g. from mapped Contacts)
- the custom objects fields are rows (attributes) of the report
- you can create CDO reports for up to 5 CDO's
- within each report there is a limit on the number of CDO fields you can use (15?, 25?) and within that I believe there are limitations on the number of text vs. numeric vs. date