I've been using an i1 display2 which has been discontinued but seems to be nicely discounted now from a variety of sources. Works quite well, but one of the problems I have is calibrating the screen brightness on the iMac as the brightness control is stepped and not continuous. There are several comments that suggest the iMac does not have the best screen for photographic purposes. I have a 20" Apple Cinema Display but when both are calibrated it is hard for my eyes to see a major difference. Have not tried any other colorimeter but this is far superior to the built in color calibration in System Preferences. I had a magenta tinge no matter what I did in System Preferences on the iMac but the i1 got rid of it immediately.
Hmmm, you're not the first to say/mention that. It beats working on images using my 15" Macbook pro, but I had some recently printed at the local printers and they were a tad dark from what I am seeing on my screen.
I've had excellent results with ColorEyes Display Pro. It's the only software I know of that allows the user to drop the iMac's luminance to mirror printer output. It isn't exactly inexpensive but it works, and believe me, after fruitlessly banging my head on the wall for months and spending lavish amounts on wasted ink and paper, it was worth it.
Whomever mentioned turning the brightness down was exactly right. I have a IMac 27 inch and Color Munki and it took some time to realize that the screen was too bright. How I finally adjusted the screen was print a picture (scenery) on my epson 3000 and simply adjust the screen to give me the same picture rather than try to get the print to match the match the screen. Duh! sometimes one can't see the forest for the trees. It took me too long to figure that one out. I kept increasing the brightness on the printer and even adding contrast. That surely was not the right thing. So now the prints are immensely better. J
i1Pro with the i1Publisher software, which is an overkill only for an iMac!
I used to have i1display 2 which I sold few days back, it did a pretty good job with coloreyesDispay Pro software.
iMac screen, while being a darn good monitor, doesn't have contrast, white point or gamma adjustments - therefore a package like colorEyesdisplay Pro should be at the top of the list if you after the best solution. If you visit their website, you'll be able to find out about the bundled offerings and the compatible hardware (colorimeters and spectrophotometers their software works with)
Otherwise most modern calibration and profiling solutions from x-rite or datacolor will do a decent job calibrating an iMac