This thread has certainly been a fun read...
Mastering can make or break a recording. There are guys in the real world who have waiting lists two years long for their mastering services. If you listen to something mastered by Bob Clearmountain you'll know why. Mastering involves so much more than volume leveling and simple eq... You can't polish a turd, but if you start with a good mix, mastering (whether your recordings are analog or digital) will bring air and life to your project that can't be done any other way.
The most simple test I can think of is to pick up an OLD copy of an Aerosmith cd. Rocks or Get your Wings are good for this. Buy it used. Then pick up the same cd, but one of the newer 20 bit remastered copies. (Or do the same with any band you like that has a cd that was remastered at 20+ bits at some point.) If you A/B them, you'll be amazed. You'll hear nuances and dynamics you never even noticed before. Any decent home stereo will allow you to hear the difference. Remember, it wasn't remixed. Just remastered. I don't know about newer stuff, but any older album that's been remastered will show a HUGE improvement. I heard parts I forgot were there...
There are mastering programs that you can use for your home recordings, but if you plan on distributing your stuff to a larger audience than friends and family, it'll pay dividends to have it done by someone who really knows the concept. You'll find that you never sounded better...
