I finally finished a first pass of the IDimager manual with some hands-on experimenting and a bombardment of questions at the IDimager forum
http://forum.idimager.com/viewforum.php?f=2&start=0. Wow! Talk about allot of info! I had no idea what I was getting myself into. At times along the way I kept debating whether the app was just too complicated and/or confusing; however, now that I'm beginning to see the larger picture, I believe it all serves a purpose and is extremely well laid out.
All I can say is, this application is everything a catalog manager should be. It's truly in a class all by itself. I have never seen an asset manager this comprehensive and yet so user friendly. If this was truly developed by one individual (i.e. Hert), then I'm totally blown away! And Hert, I don't think you have to worry about Lightroom or Expression Media catching up to the asset management capabilities of IDimager any time soon. I really can't wait to see what's coming in version 5. Simply an amazing app!
Below is a list of those features I believe are worth noting and for the most part either have more capability than EM or are not supported by EM. There were actually a number of other features that were as good or better than EM or were not supported by EM, but I did decided not to list. For example, the Stamper, powerful Batch processor, comprehensive email support, printing, elaborate CD burning, full backups, extensive built-in editor, and more). You can find out more about these in the online manual below. I did also note a few weakness of IDimager in the list (albeit not many).
I did my best to make each list item clear and concise, but again you can find more information from the online manual
http://manual.idimager.com/ where necessary.
Note - idImager refers to Catalog Sets as Catalog Labels (or just Labels).
1.  Free floating & docking panels w/ collapse capability (like PS CS4)
2.  Supports IPTC 2008 XMP extended schema
3.  Supports PLUS XMP schema
4.  Good mouse-over details of all XMP fields including XMP syntax for the given field
5.  For selected images, a per-XMP field ability to append, replace, delete, or leave unchanged
6.  Powerful script engine w/ ability to code scripts and macros directly into XMP fields
7.  Full EXIF browsing and searching
8.  Private Label support
9.  Full XMP harvesting and syncing w/ PIE adjustments
10. Visual thumbnail indication of whether images are in-sync or out-of-sync
11. Visual thumbnail indication of whether images have assigned catalog Labels
12. Visual thumbnail indicator of whether images are online, offline, or missing
13. Per-image list of currently assigned catalog Labels
14. List of most recently used catalog Labels
15. 100% loop preview from thumbnail view (similar to Bridge CS4)
16. Prevents accidental changing of star ratings when clicking on thumbnails (Use Alt-Click for ratings).
17. Full script support with library of plug-in scripts
18. Ability to setup Workspaces (including color themes) similar to Photoshop with each having a keyboard shortcut (EM2 uses View Options dialog)
19. Full image downloader w/ mirroring, validation, DNG conversion, and integrated Label assignment (somewhere between EM and IIP)
20. Full XMP GUI meta editor w/ custom schema creator
http://forum.idimager.com/viewtopic.php?t=430.
21. Ability to attach GEO location info (w/ distance radius) to catalog Labels
22. Ability to establish relationships/links between Labels for quick navigation to related Labels.
23. Supports Stored Groups analogous to Lightroom's Smart Collections, but also automatically assigns Labels as images are added to Stored Groups
24. Can drag-n-drop both directions: images to Labels or Labels to images. Can also drag directly from file system to Labels.
25. Separate Catalog Assignment mode allowing for quick series of Label assignments via checkboxes
26. No Back button or history in IDimager, but can go back to main page after a search
27. Copy and Paste Labels to/from selected Images (Ctrl-Shift-C, Ctrl-Shift-V)
28. Supports versioning for derivatives w/ Version Detection Wizard and metadata cascading across the versions. Versions show as single thumbnail.
29. Supports stacking for Pano's, HDR, collages, etc.
30. Supports virtual and fast selections for quick groupings prior to assigning permanent Labels
31. Very powerful searching capabilities including:
   Extremely fast multi-searching w/ mouse of everything including Exif, Search hard drives using wildcard characters, Boolean-wise conditional searching, Search for missing images, Search for out-of-sync images, Search across categories (e.g. catalog Labels, color Labels, file types, ratings, etc.) using AND, OR, and NOT
32. Integrated GPS with Google Maps
33. Comprehensive Light Table w/ up to 30 images w/ DNG preview support.
34. Full-screen image viewing mode w/ DNG preview support.
35. Does not support playing of audio or video files, but can catalog many different file types.
36. Browse and select images across date ranges with a visual calander
37. Extensive slideshow editor with multi-track timeline supporting MP3 and WAV. Can create AVI, GIF, and self-extracting EXE movies.
38. Extensive web publishing with powerful template editor w/ built-in HTML code and metadata access
39. Currently supports uploading to Flickr, ZenFolio, SmugMug, Google Picasa, and Facebook
40. Smorgasboard or options including complete thumbnail customization with HTML editor
41. Uses industry standard SQL database format
42. Good documentation
43. Good support forum
It is unfortunate we don't currently have a Mac version, but I would hate to see that slow down Hert's progress. Hopefully he can hire someone to help him with that down the road.
Dan