Pros: Soundstage, detail, balance, comfort, detachable cable, choice of two earpad types
Cons: bass may be light for some users
The value rating refers to the price I paid: $75 as B-stock from headroom.com.
Here is a mini-review. I have been enjoying the Ortofon O-ones for a couple of months. I bought them as a B-stock item from headroom.com for $75. They were listed as "as new" on the B-stock list, and indeed they were an open-box item that looked like new. I find these headphones to be comfortable, efficient, and easy to drive. I use them off a stereo integrated amplified, a couple of "mini" systems, and my android phone. They sound good and loud even off the phone, with 320Kbps MP3s.
I enjoy classical music, jazz, West African music, and some Italian pop. I have attended many classical music concerts over the years. My family has a subscription to the South Carolina Symphony Orchestra. Our seats are central but on the "grand tier" (a low second level) of a modern concert hall that seats almost 2000 people. Exact localization of instruments is, in my experience, not possible in the concert hall situation I described, with rare exception (piccolo flutes?). The last concert of the 2012 season included a performance of Beethoven's Ninth. This is a piece of impressive dynamic range. In the concert hall, the bass (both low brass and, especially, upright bass) was sometimes strong, but never boomy or reverberant. There was some sibilance, in the loudest choral passages. On an evening a few weeks after the concert, I listened to a recording of the Ninth through the Ortofon O-one, and I was impressed by how realistic the reproduction was. (The recording was from the boxed CD set of Beethoven Symphonies conducted by Walter Weller and published by the Musical Heritage Society. This was played using a Pioneer DV 563A CD/SACD/DVD-Audio player and a NAD 3130 integrated amplifier.) With the exception of the solo singers (soprano, alto, tenor, and bass/baritone), which were a little more forward through headphones than in the concert hall, where they were lower in level with respect to the choir, the balance and soundstage were very close to the real thing and very enjoyable.
I also compared the beginning of Beethoven's Symphony No.7 using the O-ones and the Audio-Technica ATH AD-700. The striking difference was that when the violins started up, the AD-700 placed them rather precisely left and high on the soundstage, which is an unrealistic placement. The violins were more naturally placed in the O-ones presentation.