I recently got the HD600 with the FiiO E07k/E09k. I've listened to this setup for around 80 hours now and I really think they're great!
The only other headphone in this price range I've heard is the Fidelio X1 with the Audioqeust Dragonfly (which I owned for 2 months).
I can't say that the HD600 is better or worse than the X1, they both seem to be very different headphones.
I really like the HD600 but I think there are 2 aspects that prevent these headphones from being perfect for me. Firstly, I can't 'feel' the bass as much as I would like (i.e. the bass in the X1 was way more visceral) which is possibly a result of its recessed sub bass? Secondly, I find the soundstage to be lacking (again compared to the huge soundstage of the X1).
The HD600 beats the X1 in every other aspect IMO: it's more detailed (slightly harsh treble for my tastes), is more natural sounding (i.e. good mids) and are light weight. The bass also sounds tighter than the X1 which I like.
I'd say the HD600 is better than the X1 for classical music and rock while the bass response of the X1 makes it better for RnB, rap etc. For Jazz, I find both headphones to be equally good but I might give the X1 the edge since it has a better soundstage IMO.
For movies I'd say it's a tie. The HD600 is more realistic (maybe at the expense of being a bit boring) while the X1 is very exciting but lacks realism.
On the whole I'd say that the HD600 is a better all rounder than the X1. Even in genres where the X1 outperforms the HD600, the HD600 still sounds really good.
I think my ideal headphones would be the HD600 with better bass extension at low frequencies, slightly less treble and a larger soundstage. Does this sound like the HD650 to anyone? Or maybe the HD800 (with a tube amp)?