You can migrate your raw photos, most metadata, and Aperture Previews, which are Aperture-generated JPEG using your adjustments. To get as much data across as possible, you need to select all options when converting. Adobe succinctly spells out what is and is not transferred here:
Many find "relocating masters" to an external location first. A date format or date and project format for your folders may work best. This is what you'll be stuck with in Lightroom, so choose well. The plug-in will copy all projects to one directory if the Aperture photos are managed. Not to worry, projects will at least come across as collections.
If you're copying Aperture previews, make sure you delete and regenerate them at maximum resolution!
Adjustments and "face" location on the image are the most notable data that are not transferred.
That said, some are having problems, primarily related to Aperture previews. Keywords and faces are coming across inconsistently as tags in Aperture. Most troubling, I find only 1/3 of Aperture Previews are copied. Adobe support will only go as far as helping to install Lightroom.
I suggest you vote on the following bugs to get Adobe's attention. Support from Adobe's on this plug-in seems to be whimsical, at best.