Pros - it does not affect the sound file.
Cons - it does not work.
Actaully there are 3 SoundChecks: one for iTunes playback, one for iPod playback, and one for burning. The one for burning is very poor, and the other two are not as good as they should be.
However, they do not modify the audio in the file. They use the stored sound level (which was calculated when the file was added to the library) and adjust accordingly upon playback/burning.