I've had the HD-800's for about a week now, what I appreciate the most so far is how little of their own sound signature there really is. They have this wonderful ability to change almost completely depending on the music I'm listening to, only one other phone is better at that, the SR-009's. It's nice to able to just listen to music without thinking about or noticing the sound signature that the phones are applying to everything I listen to.
The HD-800's are supremely comfy phones, their light weight really matters for long listening sessions. Build quality is top notch, hardly anyone in the headphone industry (except maybe Oppo) is even close to this type of build quality. I don't know if other companies don't give a shit (well Grado clearly doesn't) or aren't capable of this level of build quality.