From the things that drive me so crazy I blog about them department, I get and see this question all the time:
I have [insert plugin or application name] that does mastering, can't I just use that?
While nothing wrong with using mastering plugins or applications to enhance your material, you should really think of mastering not so much a tool or effect, but a series of processes that a well trained engineer performs based on a number of subjective and objective factors. Mastering software or plugins normally attempt to automate the mastering process by combining common mastering tasks to a series of presets that help less experienced musicians enhance their mixes.
Although sometimes these are very nice tools, they are just that, tools. As with any tool they are only as good as the person using them. A good mastering engineer may or may not use these tools, as what is best for one type of music will not fit for others.
So you have just spend countless hours writing, recording, and mixing your music to be the best it can be. Do you really trust a preset in a mastering application made by someone who has never actually heard your music to process fundamental characteristics of its sound?
Although it is debatable, many professionals also feel that the same person that mixes a release, should by nature not the the same person that masters it. This is simply due to the fact that if you are the one who is intimately familiar with the details of the individual tracks and how they are mixed, you will not be able to hear it objectively enough to master it. Regardless of your stance on this issue, its always good to get a "fresh set of ears" at the mastering stage and clearly mastering software or plugins don't do this for you.
So I have been doing mastering services for small bands as part of 0xf8 Studios for years now. I've done some mastering work I'm pretty proud of, and some I'm not so proud of. I never charged anyone to make money, only to have some idea of breaking even for my time. It was mostly for bands I cared about that needed extra help but couldn't afford to pay for professional mastering studios. Even still, this line of work is incredibly frustrating. Why? No need for me to type what has already been so perfectly documented. Please see this video to know all you ever really needed to know about the mastering business: MASTERING: THE MOVIE.
Last year I started winding down doing any mastering work outside of my own music. In 2011 I'm closing the doors on it for good. It's time to focus only on my own work for a while. If you're looking for an alternative to the service I was providing, check out Inner Portal Studios. He does better work and is likely a much nicer guy to deal with too. :-p