The whole idea is that the monitor looks great when you look at it, but you have no context. These calibration units (along with Datacolor, and others) put the context in. In other words, your monitor may be too bright, and you adjust for it, but the prints come out too dark. Similarly, the mind has a way of seeing what it wants. Calibrating ensures that your colors are the same as a standard (say Pantones), so when you print, or send off for a print, they use the same color scheme.
This means that the print you get will be what you see on the screen.
The units come with software that will pop up a message on your monitor that it is time to re-calibrate at intervals you can set.
I like the Colormunki better. It would also be reasonable to consider a good (though not that expensive) photo editing monitor. You will find for example that MAC's tend to be way too bright.
Hope this helps.