When calibrating a screen you have to ask what end result are you trying to match. No use having a screen in perfect calibration if you are trying to match offset printing, 6x8 colour prints, web and so on, each with their own characteristics, and expect what you see on screen to reflect the output. I doubt it's possible to calibrate a screen and have it automatically match various end uses, even with soft proofing.
A year or so ago for a large project (see
https://sites.google.com/site/tanksinker/Home/max-burns-tanksinker) which involved printing on a Xerox iGen3, I went through this process:
1. Calibrate the screen using the
System Preferences > Displays option. Took about 10-15 minutes.
2. Hold up a sample print from the iGen next to the screen and run through the various softproof options in PS until one matches very closely.
3. Repeat for several other sample prints to ensure they all match.
By spending several hours on the above I was able to match the iGen print -- colour and contrast -- with my screen so that what I saw on screen I knew I would get on the print.
It was actually a little more involved than described above because the first proofs came back with the black densities under about 5% swept totally to black. i.e. shadow detail was lost. So I selected a certain softproof option that mimicked that "too much black" look, then lightened the images so the black detail appeared, then sent off the second proof. They came back exactly as on screen. I thought the match was remarkable. I didn't think consumer monitors and high-end printers could be matched so well.
I'm now repeating the process but for scanned slides. I'm scanning my collection of slides and am trying to match the scan with the projected slide. I have built a small 20" screen out of plywood which I project onto, and that sits next to my monitor for a side-by-side comparison. I'm not sure I want to try and match the monitor to the projected slides because of one basic problem: the projector lamp, being incandescent, has a colour temperature of around 5400ยบ (according to Kodak), throwing a yellow cast on all projected images. It gives them a warm look compared to the original slide when viewed on a light box under a loupe. An on-screen match with the projected images, given that I will be projecting the scanned images on a digital projector, may well be a false match.
So, do I match the slide as is (when viewed on a light box) with the monitor, or the slide as it would be projected by a slide projector (to give the look of a projected slide)? I will probably elect to match the slide "as is" with the monitor, and if I then want to show the images on a digital projector and mimic the look of a projected slide, I will batch process them to mimic what the projector bulb does to the colours.
In summary, my thoughts on calibration are to start with a good monitor (and iMacs have good monitors), and then calibrate to the end product with your eyes and the simple tools that come with OSX.