Pros: Sound, price and build quality
Cons: Non-proprietary connector, comfort, estetics (to some degree)
Coming from a K171S followed by a DT990, I guess I wanted a combination of the two - a somewhat flat-sounding closed set of cans combined with the comfort, size and soundstage of the latter.
The DT150 certainly does the trick. Much more "alive" than the K171S, quite comfortable*, "alive" yet flat-sounding enough to handle most genres with ease. Rock, metal, electronic music, ambient and hip-hop all feel just about right, opposed to the K171S (which lacks bass and soundstage) and the DT990 (which frequency response is too U-shaped for a lot of music).
Even though the sound may be described as "dark", the bass never gets in the way of anything else while sounding full and quite punchy. It also extends quite deep, avoiding at least some of the boominess that comes at a cost with most closed, bass-heavy cans. The treble is a bit less detailed than the DT990, which results in less listening fatigue. Midrange isn't anything really special, but it sounds good, although it lacks the "magic" of the bass and treble.
Of course, all isn't that well. The bass can be overwhelming, especially on "remastered" tracks, it feels somewhat slow, and you don't get a very high-class feel: While solid built, it's still plastic and metal in its most functional form, and the looks are on par with industrial hearing aids. I like it, but it is far away from the "classic" AKGs and Beyerdynamics on terms of look.
For the tinkerers, the DT150 is very easy to disassemble, and may be a prime target for light modding.
The DT150 is at its best with a dedicated amp, but is far from unlistenable with a lesser source, though it lacks some refinement in bass and treble. The 3,5mm plug adds to the portable aspect, with a nice screw-on 6,3mm adapter included (also fits AKGs). I'm using an Argon HA2 (Mistral Audio HP-509) DAC/HA, which is admittedly not very high-end but makes the sound in general more controlled. Works well with the PA2V2 as well, though it may be a bit dark.
Recommended if you prefer a relaxed sound, don't care about size or looks, and want something that probably will last forever. The non-proprietary connector is some of a letdown, but this is due to the modularity of the headphones, there is, according to the local Beyerdynamic dealer, possible to add a microphone afterwards, where the special connector comed into play.
* Once you stretch the headband, that is. Look at a picture of someone wearing the DT150 from the front, and you see the main reason why it is so hard on the head. Unless you need max. isolation, I suggest bending the headband to a C-shape rather than the "stock" D-shape, compare with the DT990/880/770 (which is indeed comfortable). by flattening the band at the "edges" (top and over each cup), and then trying to make a C-curve, of course without breaking the band (it is quite sturdy, but the cable runs inside it).