I bought a pair of ATH-A900x (closed back) a while back and I'm absolutely loving them. I no longer have any interest in listening to my Grado SR80i and Sennheiser HD595. I hate my Grados. The Sennheiser's are good on accoustic stuff and classical, but that comprises a tiny part of my collection, so on the basis that I like the Audio Technica house sound, I decided to go for some of their open backed cans.
Ordered this morning and due to be delivered tonight. The reviews sound just like what I'm looking for - detailed, open, airy with good instrument separation and soundstage. The price point is right too, and if they satisfy me even half as much as the A900x I will be selling on my Grados and Sennheisers to fund the purchase.