I upgraded to a Spyder4Elite a while ago to help re-calibrate my two Apple 27" displays - one a LED Cinema screen, the other a newer Thunderbolt. The initial calibration was very successful, however when I tried again a month ago I found that the calibration seemed perfect except that the neutral shadows were all messed up - they had a red / magenta cast to them.
I tried a full calibration, changing the brightness of the room etc and nothing helped. The problem was especially bad when using Lightroom, as all its panels lower than about 127,127,127 RGB would display a horrible red / magenta cast and not be neutral. I even took photos with my camera set to manual white balance to prove this.
The solution was quite simple in the end - I usually set my display brightness quite low as the ambient light levels are low and this reduces strain on my eyes. However, it seems that at these low values display calibration can mess up the darker parts of the display brightness range. So I bumped up the display brightness on both screens to 120 cd/m2 and used that as target too. Once calibrated I lowered the brightness on both monitors to a comfortable viewing level, and reviewed the neutral shadows. They were perfect to my eyes.