I've been following Shotwell, because I'd really like to get a less-fat image organizer that digiKam. What digiKam does well:
* You can tag pictures, and captions, and everything is stored INSIDE the picture (I've been burned by this before, using Gallery to write captions for thousands of images and then, all of them lost).
* It's FAST (that's important, I've got thousands of pictures, I want to zoom through them and delete, delete, delete (or watch, watch, watch))
* It doesn't copy or change the pictures
* It does FAST searching (how? well it has it's own indexed database ALONGSIDE the pictures, it's basically just copied the information that already exists inside the images, but done so because it needs to search it).
You can use Tracker to do the searching and indexing-part. I've heard it's much better these days.
The other thing digiKam doesn't do is having an easy way to tag pictures fast (you have to click much with the mouse!).
Anyway, thinking it's way cool you're considering changing the way the software works! :D