Cons - Limited sound stage, expensive, light but uncomfortable on the ear pads.
I tested these headphones and was not very impressed with them. The price was very high for a used pair of headphones and it did not seem to compete well with other brands at lower price points. The sound was very clear, probably the best I have heard in any headphone yet. The weight was so light you could barely tell they were around your head. The sound stage was not very present and I was expecting more. My ears seemed to feel scratched in the ear pad opening. The chord is amazing, good durability, looks and feels good. Did not really enjoy the whole stax experience and the adapter price adding another 100 dollars. Comparing to other headphones, I prefer my hifiman headphones to these.
New headband assembly (clicks in places instead of sliding - doesn't slide off on its own), new driver assembly (no more glue - now a resin enclosure), new driver materials, leather pads (nice thick ones). A manual with English sections! (no, seriously, it's a feature )
Whether the new arc assembly+pads are more comfortable than the trusty old Lambda (classic) frame/old pleather pads is debatable. Still they're certainly pretty comfortable in their own right.
Unlike with Lambda frame HPs I owned, with SR-507, you can now eat or chew while wearing them. Uuuum, not that it helps resale value, or that it's a good practice to have food/drinks near $900 HP.
With respect to the new driver and driver enclosure, it looks like a good idea too (vs glue)
The new Click assembly doesn't slide from the set distance between the arc/suspended headband bit.
Sound (general) (TBC)
So far - not fun headphones like SR-404 (404 had boomy bass, mids emphasis, to me - very enjoyable for some music, but might not suit some tastes and  isn't as detailed as say SR-202). SR-507 bass goes deeper isn't boomy/is better defined. Mids aren't as prominent. I find I can listen at lower volumes than SR-404 and still enjoy the music/hear the details (probably because of the less boomy bass and less emphasised mids).
I really can't put my finger on the sound signature. They seem to not have any overly prominent/annoying or dipped or FR chars like my previous lambdas. Mids are still a bit warm, and the detail level is great across the FR.
They're certainly feeling more balanced and neutral than CD3000 or SA5000 or SR-404.
I'll put more impressions here when I have more time to listen to them.
The arc is extremely uncomfortable for me to wear for longer than about 2h. While it is a lot sturdier and doesn't crackle when twisted (when you move your jaw or head or ears), the pressure at the top of earpads gives me a headache.
I've replaced the arc with a Nova Classic old style arc assembly (sr-303/2/4 ones will also do, thank you, spritzer!).
Sound suitability to specific genres of music (TBC)
Generally suitable for most music genre (provided you're ok with neutral to slightly bright presentation). The deeper bass and neutral highs make them good for anything from classical strings, piano solo, orchestra, voice to subtypes of rock and metal(ummm yea, not really... see below) (including the harder types). Just make sure neutral is what you want, not bright and fun/boomy bass. (although, they are reasonably bright, but not as much as SR-404 in the mids or highs (no mids etch))
I also had a chance to compare them against 404LE at the melb meet. 404LE had a bit more forward and fun mids and bass, and was probably more suited to easy listening. (so more similar to 404, I suppose). 507 was more towards the neutral side, and probably had a bit more bass extension, but a bit less 'fun' (amount) bass.
a side-note on bass - if you boost FA-002w's lower end by about 2db on 30hz, 1db 50hz, it goes even deeper than before. In comparison, FA-002w's bass is probably about 40-60hz deeper (and +8-10 db louder) than 48e and 20-30 than SR-507 (and 4-6db louder). It's interesting to see what's missing from DT48e and SR-507. Does it make a difference? Not so much for classical, but for metal, rock etc certainly =\ For classical for old recordings it's better to have brighter mids (to make out what's actually going on ). But for easy listening, probably would be nice to have the lower end too :\
A word of caution on the new headband:
About the screw at the bottom of each earcup holder.
Well, one of the screws on my SR-507 came off today (the middle one that actually fixes the holder to the headband). As did one of the earcups...
Now thankfully it happened when I was playing with them on a table on a bubble wrap pad, but I don't image it would be a pleasing experience when one of your $1k ear-speakers falls off your head.
It took 1/2 a day to get a proper tool to put it back (turned out it's a star screwdriver not a hex tool...the right size star screwdriver head is marked cr-v t6), and then a few minutes to unscrew the other cup to see what the proper order for the stack of:
rubber thingy, metal thingy, metal tube, [holder] metal thingy with a smaller diameter was...(I still don't know if it's the right order, since the stack fell apart when I took off the second earcup before I could note the order)
I'd really recommend you check that your screw is tight and you won't have one earspeaker flying across the floor when you try put the hp on.
Now I'm slightly wary about how durable the construction of the headband is and whether I won't have a cup holder bolt snap into the holder screw slot and necessitate a new headband that cost > $150?