Pros: Outstanding mids, absence of sibilance, huge soundstage.
Cons: Need for powerful amplification; hard to find, increasing price.
After the great reviews by John and Ed, there is really not much to say.
The 009 is to my ears, a perfect headphone, although at times in the brighter side. In spite the 009's however, I frequently grab the Sigmas/404 for chamber music, vocals and chorales.
The bass can at times get a little boomy in complex orchestral passages, but the wonderful mids give piano, voices and small acoustic instruments groups a relaxed and realistic sound you can listen to for hours.
Certainly a specialty phone, and one I will never sell.