Silver Efex Pro has a straight forward interface similar to Lightroom and Aperture that helps you move through the conversion process quickly and easily. To launch Silver Efex Pro from Lightroom, choose the photo to edit, go to the Photo drop-down menu, and select Silver Efex Pro in the Edit In option. A popup will appear (shown below) with options for what to edit, file format, and whether or not to stack the file with the original. Once you click Edit, you are greeted with a minimalist interface (shown below) that slightly resembles Lightroom.
The interface consists of a list of presets displayed on the left and the tools and filter on the right. Your image is displayed in the center with a choice of three different background colors. Before beginning to work in Silver Efex Pro, I recommend watching one of the various quick tutorials available at their website. The tools and filters are very easy to use and are laid out similarly to Lightroom or Aperture where you are meant to work from top to bottom to go through the editing process. Some of the filters I liked were the Color Filters which simulate placing a color filter over the front of your lens and the Film Types which allows you to choose certain looks to resemble certain film types.
Once your edits are complete, click Save at the bottom right and the file is saved back into your Lightroom library. Silver Efex Pro closes automatically and you are returned to the Lightroom interface. Overall, I think the user experience of Silver Efex Pro is extremely smooth and you hardly notice you are working outside of the native Lightroom environment. Since I am not an Aperture user, I cannot vouch for the user experience with Silver Efex Pro on that platform. However, based on the Aperture tutorial at the Silver Efex Pro website, it appears to be equally smooth.