Pros: Detail, naturalness and resolution on another level above anything else out there.
Cons: You'll never have a good enough source for them and the best rig will cost as much as a quite excellent speaker rig or even a car.
Similarly the 009s have had such an effect on friends who have listened to them. Much like the Omega IIs and a suitable high-end rig, it is possible to make out the form of the studio music was recorded in by the detail of the echo off the walls. Listening to a classical concert I can even hear the detail of the waves of echos from the instruments coming off the walls. With these earspeakers, as Stax terms them, it is as if everything is simply on another level compared to dynamic headphones. Only the more recent high-end orthos, such as the LCD-3s and Hifiman series and maybe the HD-800s, Sony R10s and my Symphones Magnums come anywhere close. Now the SR-009 takes all this to yet another level. In the same way the first time I tried Stax and compared them to my regular headphones everything was so much more clear and real, the 009s have achieved this over even the orthos, which I hold in the highest regard. Questions about resolution, harshness, distortion, ringing and other issues simply seem not to exist.
While Stax will never give quite as much impact, or slam to music as othos can (while maintaining a high level of detail), the orthos, already awesome in this regard, especially with something like a high-end hybrid amp, wont give you instruments and voices with as much clarity as 009s, if they are arguably better all-rounders. The limits are only set by your source and amp, especially the former. I could easily visualise a $25k system with a pair of these (along with a Liquid Lightening or Blue Hawaii SE and Esoteric K-01 DAC).
Tonality-wise they are similar to the old-style Omega I and Lambda, that is, quite a bit brighter than the Omega II, which was always the odd one out in terms of sound. They suffer, like other models, from the "Stax fart" where moving your jaw and breaking the seal causes the protective membranes to make a sucking sound. However, like the original Omega and unlike the Omega II, they seem to be less difficult to drive, with detailed bass even from the older amps, such as the T1S. A good combination I found was the NOS Metrum Octave DAC, which is relatively mellow but still detailed, along with the Stax 727A amp. This gave a number of people an OMG moment listening to acoustic music, though it was not quite the ticket for rock. The bass, like the power output of a Rolls Royce, is "sufficient". The LCD-3 I found more entertaining in that regard.
They are again similar to the Omega I in that they come in an elegant wooden box with the nicest foam of any box I've encountered in recent memory. Opening it up, I was greeted with a "new car" smell, suggesting they source the leather from a factory that makes car seats. The headband has a new 10-click (per side) adjustor, which is quite stiff to adjust but holds firm unlike the old Lambda headbands. Beyond that, the fit is typical Stax -- light and comfortable. The aluminium design is of the highest quality, if it does seem to contrast the rather simple headband arrangement. Like its maker, it is in many ways an understated design, its statement in its perfection of design and sound.
So, ultimately, we have our king. Long live it and its maker I reckon.
(Updated June 23, 2012)