See this post for a coverage of issues:
Furthermore: The "right" approach depends on what you want in your final result. Do you only want originals? You say only raw files, but no original JPEGs? What do you want the Catalog's remaining file structure to look like? Do you also want Aperture's previews? No? Without them, you'll lose all reference of how you had edited the photo.
Do you want Lightroom catalog to convert as much as possible? As much as advertised? Even there you'll have problems. Faces and keywords not coming over consistently, and only a handful of Aperture previews come across. Personally, I'm going to wait indefinitely until these issues are fixed.
There is a 3rd party plug-in that exports from Aperture, but it seems to be inconsistent as well, and seems to do less in more steps than Adobe's plug-in.