I'll also chime in as the original poster was nice enough to come out twice to work on my RS40 and did a very good job. He had originally talked me out of buying a D2, and with good reason. However, by dumb luck I found an unused and unopened i1pro sitting in the basement at work and was allowed to borrow it, pretty much as long as I want. Now, we were able to get very good grayscale results with C6 and i1pro combo that Doug has, but gamma was a bit screwy (due to the display itself, not Doug or his equipment). It's running around 2.5 down below 20% and 2.1 at 80%. Eyeball correcting this using the gamma controls produced mixed results, so I had the idea that I could get a cheap D2, profile it against the i1pro and use the D2 only for gamma and for grayscale at 20% and below where the i1pro loses accuracy.
Having bought a unused D2 off ebay for cheap, I did some quick comparisons to the i1pro last night. I don't have the charts, nor did I save them....it was late. I only looked at gamut and then grayscale at a few points (80, 30, 20, and 10). The un-profiled D2 was way off. Once profiled, it produced very similar results to the i1pro except at 20% and below...here it was close, but who knows which was actually closer since the D2 can read lower light levels than the i1pro. I'll use the i1pro for the rest of the calibration
I guess what I got from this that the D2 might be better than no calibration at all.....but if you had a display that was close OOTB, you could actually make it worse. I would not trust it. Now if you are me and stumbled upon an i1pro to use for free, then a cheap D2 might get you pretty good results as once it was profiled, it was much closer to the i1pro. I intend to get a C6 for Christmas to go along with the i1pro. But for now, a profiled D2 might help me correct my gamma issues. If not I can sell it for what I paid on ebay!