One thing that almost everyone agrees on is that investing in professional mastering is worth it if you intend to actually sell your CDs to the public. Mastering is not re-mixing, but more like "super-normalizing" so that all the tracks on your CD have similar loudness and sound compatible with each other...but there's lots more to it than that.

There are two FAQ articles you must read on mastering...here's one link and here's the other.

Reader Experiences

I had an opportunity to spend four hours at one of Boston's leading mastering studios last night. Interesting experience. The "funniest" thing, though, was that of all the equipment in that place, I did not recognize a single piece! Many of the company names, sure...Lexicons, TCs, Sonys, etc., but the boxes themselves, no. When you go into Guitar Center and go to the "pro audio" deparment, turns out that's actully "amateur pro audio." I certainly didn't expect to recognize everything there, but zero devices? Wow.

We were getting professional help on a "finished mix" that really stunk...too much drums, guitars unbalanced, all kinds of problems that we didn't have the freedom to go back and change. This was a soundtrack on DAT from a video shoot we had done. Mixed live by an idiot.

We tried by ourselves to improve the sound using EQ and compression but couldn't get a good result. The mastering guy worked a miracle for us, no exaggeration. The video will be done in a few weeks...
-- Marc Hertzberg, HELP!