"Without losing anything I already have" is asking a bit much. It's not as if Apple are likely to have helped Adobe create the migration tool, so it's more like getting off the Titanic with your suitcases but maybe leaving a few things in the wardrobe. The key stuff like projects, albums, star ratings, keywords and other metadata will arrive in Lightroom, but things like smart albums won't. You need to download the Lightroom trial and try the import process for yourself.
John's depiction is accurate. Many still have issues, and others, like myself, have been able to get the importer to finish libraries with tens of thousands of images. However, the fact that the importer finishes without errors is not an indication of success. I've had issues in my tests with inconsistent results with "face" tags, and not copying over half of Aperture previews.
If you have the same issues, vote on them so they get more visibility. Think most of the attention now is going to failures causing the importer to halt.