BryonD (the programmer) and I each spot-tested it against a wide range of MM creatures and found it very accurate. For the most part the differences will show up in the rounding-off math, and at very high CRs (in the CR20 range).
A Pit Fiend, for example, clocks in at the high end of CR22. If your experience is that a party of four 20th level characters finds a single pit fiend to be "Difficult" and not "Moderate" then you will find some comfort in that CR calculation.
Ultimately, if your definition of "accurate" is "Does it match the MM exactly?" then I'd say you'll be satisfied. If your definition of "accurate" is "How well does it predict a monster's CR/EL?" then I think you'll be very satisfied.
With regard to Dragons, this is a perfect example of accuracy to WOTC vs. accuracy to actual play.
We do not continue the (in my opinion, ridiculous) tradition of WOTC in under-costing dragons in order to make them seem "tougher." This is a known and admitted bias on the part of the WOTC designers, but it defeats the purpose of the CR system-- the dragon fight ends up being tougher than expected AND the PCs get less XP for it.
If you want to calculate your dragon CR to WOTC standards, take the value shown at the bottom of the page at "Golden Rule" (cell C284) and divide it by 2.
(The next cell down, "Monster or Character" (cell 285) performs the normal 2/3 calculation here-- ignore this cell and do a 1/2 calculation instead, for Dragons.)
Wulf