I'm talking to myself again here, but that's never stopped me yet....
As reported above, the first results from the Spyder3 Pro (which, by the way, is now branded 'datacolor' rather than 'Colorvision') were OK but not over-inspiring.
The Spyder3 software lets you either check the current calibration or recalibrate. I did a few checks and it told me all was well, but there was still (to my eyes) an issue with a slightly uneven colour balance over the tonal range. Mid-tones were, if anything, slightly warm (yellow/red) whilst extreme highlights went the other way (towards blue/magenta). I should say that these colour casts were *very* slight, but I'm something of a perfectionist (otherwise known as a fussy bu99er).
So I did a complete recalibration and, lo and behold, the results were distinctly better. This suggests that...
(a) These things can work.
(b) But not every time.
(c) They are not absolute. You can't rely on them to get it right, and they don't necessarily give the same results every time.
Overall I'm fairly pleased with the Spyder3. It has (at the second attempt) produced a result I'm 90% happy with, and short of paying at least twice as much for a monitor and calibration hardware/software I don't think I'm going to improve much on that.
So a qualified 'Recommended' from this reviewer.