I have been mastering for a long time and Steinberg WaveLab 8 is the only software I use for mastering now. It does everything I need and provides one of the best user interface experiences period. The graphic spectrum display and phase meter provide extremely useful visual feedback. If you're interested in a professional program to master, Steinberg WaveLab is one of the best.
If you're looking for an all-in-one, professional mastering software, go this direction. I have been personally using Wavelab now for over ten years and will never change. All of the analyzing tools are outstanding with the ability to master from a graphic standpoint as well as an audio standpoint. Complete with speaker management and a host of great sounding plug-ins! Highly recommended!