I had the Centaur 09 groupset when they were first released and found that the right hand shifter (rear derailleur) with respect to downshifts (small to big cogs) were vague and found it impossible to get the rear derailleur to shift properly (it would shift well in one direction but not the other). Finally in October 09, I decided to ditch the Centaur 09 and replace it with a parts bin Chorus 09 (shifter, derailleurs, cassette and chain). Chorus 09 was a huge improvement - rear derailleur shifting was flawless and fast. Downshifting had clear distinct clicks unlike the Centaur 09.

i don't know if the more recent releases of the Centaur 09 has improved over the earlier or first release, but if it has then you can probably disregard my comments above.