1. Because of natural variations in parts, each monitor comes off the assembly line displaying slightly differently. This means the stock monitor profile that ships with every Mac will be a little off for each monitor, so that it's a good idea to make a custom profile by calibrating.
2. The OS X calibrator is like any other software calibrator in that its accuracy is limited by the fact that human eyes are very subjective and are easily misled. Even Expert Mode is not good enough for a demanding color pro.
3. A custom profile with a hardware calibrator is the best way to get the best color you can out of a monitor, because the sensor will objectively measure how the monitor is producing every shade and color, and then will build a custom profile for your specific unit.
4. After you calibrate, your monitor will be displaying colors as accurately as it can. However, you may be specifying colors that cannot be reproduced by a specific printing process. If you have a profile that represents that printing process, you can improve the color representation using the Proof Colors feature in Illustrator. This will not make colors look "better," only closer to how they will actually print under the limitations of the ink and substrate.
5. If colors are much lighter on the screen, it is common for a monitor to be set way too bright. Monitors are a lot brighter than paper, for instance. On my 20" Cinema, I get good print previews with the monitor set to 90-110 cd/m, which translates to about one square when you hit the Brightness button. Not sure what 100 cd/m is on a 30", but this can be measured and reported by a hardware calibrator. Many color management pros complain about the over-brightness of today's monitors and how hard it can be sometimes to dial them down to something that resembles what we have to print on.
