Pros: Detailed. Smooth. Balanced. Refined. Excellent build and comfort.
Cons: Expensive! Sound does not match the SR-007.
Compared to most of the competition, this is inarguably a wonderful headphone and I do enjoy it. But sonically, it does not compete with a well-driven O2 mk1 to my ears. I simply find the O2's richer and more involving tone to be more natural.
The 009 is more alike the Lambda series than the Omega series. It's no harder to power than the Lambdas and the sound signature is reminiscent, but probably a step up in refinement, soundstaging and bass extension/impact. In general though, the sound is typical STAX. Natural, agile and very detailed. But then again, compared to the O2 mk1 it falls short. Particularly in the midrange, which lacks the organic presence of the Omegas.
The build quality is excellent though and certainly on par with what you can expect for your $. Precision-engineering all the way through; looks fantastic and feels substantial. Super comfy to wear as well. I have no issue with regards to the fit & finish of this headphone - it looks and feels like an expensive, high-end product.
But with a price that's pretty much DOUBLE that of the O2s at this time, I do not consider this headphone to be a very solid investment. The O2 is more demanding in terms of amplification, which can get expensive, but when driven to it's full potential the O2 mk1 just sounds better to my ears.
Just my 2 cents.