Jeremy;
Leaving Aperture is possible but there are some complications depending the type of image files that you originally imported. I started moving my Aperture Library of 32,000 masters last weekend. I have been busy all week so I plan to continue working on that later today. I have decided to work with Lightroom and Microsoft Expression Media (iView Media Pro) for now.
Here is what I have learned so far.
File type = supported camera raw: This is the easiest situation. Use the Export > Save Masters command and select the 'create XMP sidecar' file option. Both LR and iView will read the sidecar file.
File type = JPEG, TIFF, PSD, DNG. The problem here is that although Aperture will create a valid xmp sidcar file for these files LR and iView expect these files to have metadata embedded directly into the file so they don't actually read the sidecar file. Aperture exports an exact copy of the file you imported. The result is that the metadat that was in the file you imported is still there on export but any changes, ratings, keywords that you applied from inside Aperture wind up in the xmp sidecar and get ignored.
Everyone will suggest using Annoture to sync from the Aperture database to an iView catalog. However Annoture is not running on my computer under Leopard. I think that changes to Applescript to expand multi-language support may have broken the Annoture script for Leopard users.
Likewise there is a good export plugin from Josh Anon that will add Aperture's custom metadata fields (if you have any) to the XMP sidecar files. If you haven't created custom metadata fields then you probably don't need to use this plugin. The plugin seems very unstable for me under Leopard. It won't process more than about 50-100 files at a time. Very impractical for a whole library export.
When I moved into Aperture I wrote my own Applescript to transfer metadata from iView to Aperture. If nothing off the shelf works I may decide to spend the weekend updating and reversing that script.
I think that Aperture's a great program. I hope that I may be a user again in the future. I just couldn't get it to run at useable speed on my up to date system. LR by contrast is going much faster so far with the test catalogs I have been working on so far.
If you need to contact me offline then use my website contact link:
http://www.thomaswhitephoto.com