As a Mac user I've tended to rely on the standard display profiles provided with Apple's computers and screens - although aware that they weren't perfect, they were close enough - and sufficiently consistent - to predict end results.
However, I purchased a good quality third-party 27" IPS display for use with one of my Macs and the colour consistency was very poor indeed - trying to calibrate by eye was pretty much impossible.
The Spyder4Pro is capable of handling multiple screens and was therefore the obvious choice. After a little experimentation, I finally managed to calibrate the new screen to be a perfect match to the iMac that was driving it as a secondary display. The iMac's built-in screen was also much improved (it had been a tad too cool previously).
If you're serious about colour fidelity (whether for photography or DTP), this is the best £100 you're ever likely to spend. Certainly it has improved consistency in my own workflow considerably