Mastering is the final trip for most singles and albums before heading to the duplicator. But like choosing a producer, or a musician, or a studio, one should listen to previous work by a mastering company before turning over your work. I have paid a lot for mastering services and been very dissatisfied with the result, because the service seemed to have no "feel" for the genre of music they'd been given. I don't think everyone offering mastering services is suitable for every genre of music. For this reason we've started doing our own mastering and have been happy with the results. There are books available on the process and you don't need tons of equipment to get the job done. I know engineers who master album projects in Sonar Producer, for example, by importing all the tracks onto one stereo track in a new project and then apply mastering tweaks with the software and plugins included with the DAW. And what I've heard was impressive.
Like everything else with music production, get educated about the process and LISTEN to the final product compared to other work in the genre. Have your friends listen, your enemies, your ex, and your dog. It is an extremely subjective art in the end. My experience in doing our own mastering is that less is definitely more. Fiddling with the final product too much is the kiss of death. My favorite program for mastering these days has been around a while - CD Architect from Sony.
Just sayin'

Bob