Yeah, I used to use a Spyder 2, and then when I got my iMac mid 2010 it got an eye 1 display and I actually really liked that unit. So when I got the rMBP it came with Mt. Lion loaded, which doesn't support the ey1 display, so I would have to get the eye 1 display pro. I know this is a good unit, Spyder 4 I will look at. I'm familiar with their software they both let you do a before/after comparison. I think the Spyder 4 may be a newer design than the I1pro so on and so forth. Thanks to you and Schwang.
Now let me offer a good thread to look at regarding the various strengths and why some professionals, but certainly not all like one over the other. A lot of it depends on the software incuded and its ability to let you choose native white point which is of varied importance to some. They all give you choices of the standard 5500 or 6500 or D55, D65 (similar but again technically different.) Some of the software allows you to actually choose the temperture, ie 6000, 5998 etc. I remember at the photo department of the school I was going to, they were calibrating their monitors and printers with supporting viewing stands, at 6000K. They even generated profiles for populer papers to work at 6000K. So you see the calibration process extends beyond just the monitor. But stay tuned. Some of the software allows you to also profile your printer, and other devices, ie projects etc. The I1 models such as the display pro allow the most choices but are more expensive. Choosing the white point is real important to matching screen to print, but some believe that the basic settings all the programs provide are quite sufficient. Profiling the printer I think used to be more important as in the example above of a 6000K white point. (some will disagree) but now if you are printing yourself most of the papers have pretty good print profiles with them and you don't need to profile the printer so much if you are using standard white points, and don't forget, the eyeball comes into play as well. Point of calibration is to match screen and print so you can cut down on the ink and paper you have to use to get the print that matches the screen. I know some pros that never calibrate, use an iMac out of the box and do it so much that they eyball huge prints with no test prints. Anyways, some high end professionals, especially in both commercial photography and video where really precise color reproduction for files that are getting passed around, or that used specified color for printing, such as Pantones etc. really require the high end and exact color matching. For instance if you are doing a Marlboro read commercial where you will show the pack, you had better have the precise red. (even though the print houses will also compensate) but it saves time and money if things are matched up to begin with. The exciting thing about the rMBP monitor is that one, because of its color depth and color gamut range, it is one of the very few lap tops where the color can be calibrated enough to use in pretty serious corrections. This is something the older or current non-retina displaysreally don't do. You will see lots of threads around on photo forums that talk about how most laptops, even the beautiful MBPs were ok for working in the field, but not for really accurate color and tonal processing. That has now changed (if they would just get the screen problems worked out).
Check out this thread on Photo.net, It gives a good basic rundown and a couple of guys on this thread really understand the physics of calibration much, much better than I do. Sorry for the long/windy post.
http://photo.net/digital-darkroom-forum/00aTQV