Equipment

 
Spotify Premium
Firestone Audio Fubar 2 DAC
Firestone Audio Cute Battery 2 amp
 

Build and comfort

 
I upgraded to these from Audio-Technica ATH-M50s. I very much like the sound of the M50, but found that their weight made them uncomfortable to wear for very long sessions (i.e. all day at the workplace), so most of the time I would use a pair of Panasonic RP-HTX7 because they're light and very comfy, despite being sonically inferior.
 
The TH-600 has very good build quality. The cups are large, understated, and have nice texture. The size adjustment mechanism gives a very satisfying click. The pads are light and soft and seem to resist oil nicely. They are not as light nor as comfortable as the HTX7 or other light, comfortable headphones like offerings from Bose, but are certainly lighter and comfier than most. They clamp lightly, so leaning back or shaking your head vigorously might cause them to fall off.
 
Overall, they are some of the most comfortable headphones I've ever had, and I have no problem wearing them all day.
 
A note about sound isolation: though these are closed headphones, they do not offer much isolation at all. However, they do not leak much sound, either. If sound isolation is important, then these definitely do not fit the bill.
 
The cable is solid and braided, but unfortunately is not removable. As others have said, this is probably the TH-600's only considerable downside.
 

Sound

 
Disclaimer: These are my first higher-end headphones. Other headphones I've owned include the aforementioned ATH-M50s, AKG k501, AKG k271S, and Koss KSC-75. I found the sound of the M50s to be very enjoyable, especially the bass extension, despite them being too heavy in the low end.
 
The TH-600 goes even deeper, has better impact, and yet the bass is more controlled. But the highs are where the TH-600 really shines. They are detailed and crisp, without being too harsh. When I switch from the TH-600 to any other headphone, cymbals, high synths, and shakers seem to fade in to the background.
 
Soundstage is another strong point with the TH-600. It's much wider than any other closed headphones I've ever tried, and separation is great. They don't sound like a closed headphone.
 
The midrange is the TH-600's weakest point, as it is slightly recessed compared to the bass and treble, with vocals sometimes lacking body. I don't usually delve in to EQ, but it seems to me that these might benefit from a small, well-placed midrange bump.
 

Summary

 
These clearly trounce the M50s, as they should, considering they cost more than twice as much. They are quite light and comfortable. The bass is tight and strong, the highs crisp and clear, and the mids slightly thin. I love them, especially for the money. The only reason I haven't given full marks on a few of the categories is that I know that there are other, more expensive headphones out there that are superior, such as the TH-900.
 
People have been raving about the value of this headphone since back when it was $800+. At the current price point of ~$500, it seems like a no-brainer.
 
I'll be picking up a Fostex HP-A4 DAC/amp in a couple weeks, so I'm excited to see how that will pair vs the Firestone stack.