My initial thoughts on the X1 after the first few days:
Things I like: well, most everything about the sound. I had specific expectations and it's pretty much lived up to it. I like everything about my HD650 -- the warmth, mids, comfort, and laid back treble presentation, but I do wish there was a bit more sub bass. I like the TH600 for its great bass impact, but the treble is more forward than I like. So I was hoping X1 would combine most of what I like about the HD650 and add the TH600 bass. What it delivers feels pretty close. The bass impacts more and extends further than the HD650, almost on par with the TH600, although it does sound less controlled and there's some bass bleed, but it doesn't really bother me. I have to listen to more vocals for the mids, but no shortcomings stand out yet. The treble presentation is what I was hoping for, laid back and not at all forward. They make the TH600 sound bright and cold, which I'm not thrilled out but that's not the X1s fault. Soundstage is the same as mids, need more time to focus on that. So overall very happy with the sound. I'd sacrifice a touch of warmth for a touch better bass control, but that's a side point.
Things I don't like: well, most everything about the comfort. I don't have a large head, but I still found the elastic headband adjustment to put too much tension on my head. Also, the side clamping on my ears is stronger than I like. The HD650 clamp can be removed by a bit of headband bending at the metal and mostly through rotating the drivers back so they sit flat against the head all around the ear. But the X1 can't really be stretched like that and the drivers only pivot in one direction. Both my HD650 and TH600 pivot in both directions to achieve the best fit. Also, ear pads are too stiff and have scratchy velour. That's not a good combination with high head clamp. At least Beyers and Senns have very comfy velour. I don't like the ear pad smell either, it's this "cheap factory chemical" smell than none of my other headphones have had. The cord and 1/8" adapter are pretty lightweight and cheap feeling. I replaced the cable with a Mediabridge one, but I would eventually make a nicer one.
So, the big question for me is: will I keep the X1? I don't know yet. Sound wise, yes, it's a keeper and my TH600 is feeling less secure. But comfort has equal footing to sound in my book, and the X1 does not deliver in that department. If I can somehow get acclimated to how they feel on my head, they'll stay there. If not, they're out.