Plenty of information on this on the Adobe Blog, Victoria's initial walk-through, and forums for issues.
But to simply answer your question, yes it will migrate originals. Adobe advertises that it will export Aperture previews, but after several tests, I haven't found that it works as advertised for previews:
Aperture stores your masters (what everything is based off of), thumbnails for viewing quickly, and previews. Aperture previews are final renditions of your edited photo, at the resolution you specify.
Lightroom has no way of migrating your image adjustments to lightroom--just the final resulting image in the preview. And I have yet to see it migrate more than 1/3 of them.
Before you begin your own tests, I suggest you set your preview preferences to full resolution (not 1/2, or a set resolution). Create smart albums to help you then create new previews and delete previews you don't want in Lightroom.
I'd also suggest you use the "relocate masters" function, particularly if you have any "managed" masters, to store them in a file structure you will want viewable in Lightroom. (Unlike Aperture, Lightroom exposes the underlying file system in one view.)
Then run the plug-in, make sure you enable the option to copy previews, and then use a utility like DIFF or ARAXIS afterward to compare your master and preview directories in Aperture with the resulting images in your Lightroom Library.
Please be aware that the Adobe plug-in will NOT transfer any geotags you have made within Aperture. This is because Aperture does not write gps coordinates into the original files but only stores these data within the Aperture database. This issue affects both jpegs as well as raw files.
The workaround is to avoid using the Adobe plug-in and to export your files from within Aperture:
Raw files: Export original with metadata in xmp sidecar file
Jpeg files: Export version in original size in best quality (low compression)
This way all your geotags and other metadata can be transferred to Lightroom. The downside is that you will not be able to automatically transfer your Project, Folder and Album structure over to Lightroom. When you export both jpeg and raw files from Aperture you can however easily organize your files in your Project structure by selecting "Project Name" in the "Subfolder Format" within the export window. Your "Projects" from Aperture will in Lightroom be "Folders".