Build/Durability: The build quality on these headphones is quite good. They are very firm and sturdy, but not very flexible. I can't stretch them a whole lot without feeling that they might break. However, this sturdy build and inflexibility plays an important role on isolation, which I find to be quite stellar. One thing I absolutely hated about these headphones is the coiled cable. Out of the box, they provide a 3m coiled cable that is just too heavy and annoying to carry around. The way I fixed this was by using the cable winder that came with my HD202s. I uncoiled part of the coiled cable and winded it with the cable winder, so that the coiled cable is much shorter and not as heavy as it was out of the box. By doing so, I have no problem clipping the cable winder on my shirt or just leaving it hanging, and its much more lighter and portable. I highly recommend getting a cable winder and uncoiling the coiled cable for anyone who owns an HD380 pro.
Noise Isolation: The noise isolation on these headphones is very good. I have worn these outside and they blocked out most sound, but it's nothing close to the noise isolation you'd get on an IEM. But for a closed headphone, these are one of the best for noise isolation, and although I do not own the HD280 pros, I hear that there is less clamping than the HD280s, making them much more comfortable. For the most part, I am unable to hear any outside noise, but then again I play my music quite loud.
Comfort: At first, I found these headphones quite uncomfortable because I haven't worn a closed headphone with this much clamping force. However, after a day of wearing them, I got used to them, and suddenly found them not only necessary but also a bit comfortable. I would describe the clamping as not being too tight around the head, but just enough to isolate very well. The pads are great for blocking out noise and not leaking sound. If there is too much clamping force, once can switch out the stock pads with velour pads(they fit HD5x5 velour pads). The velour pads open up the soundstage quite a bit and make the mids and highs sound much better, while sacrificing a bit of the bass. Also, you lose the sound isolation with the velour pads. I prefer the stock pads, but if comfort is a problem, switching to velour pads is a quick and easy solution.
Sound Quality: Out of the box they sound very much like the HD555s and HD595s in a closed form. They are very detailed, precise, and balanced. I have heard that there is an improvement on the bass from the HD280 pros. Though I'm not a basshead, I find the bass to be quite sufficient in its amount. What I find quite good on these headphones is the mids and highs, which are very detailed, making an overall analytical sound from the HD380 pros. I listen to mostly classical, jazz, hip-hop, and a little bit of rock. Also, my source is a Sony X1051 amped with a T3 portable amp. I have only tested it with an amp. I will give impressions of these unamped if anyone is interested.
Comparison with HD595, HD448, IE7, and IE8: In comparison with other Sennheiser models, I find the HD380 Pros to be sounding mostly like the HD 595s, followed by the HD 448s, then the IE7s, then the IE8s. They are very close to the HD595s because of it's similarity in the mids and highs, with a decent amount of bass. The HD448s focus mostly on its powerful mids, but they differ from the HD380 pros by the amount of bass, which is lacking in the HD448s. The HD380 Pros are quite similar to the IE7s in that both are neutral and balanced, but I find the IE7s to be more focused on the upper bass/lower mids. They are least like the IE8s in that I find the IE8s to be overpowering in bass, which produces a veil over the treble. The HD380 pros have an outstanding treble, and an overall neutral sound.
I will continue using the HD380 pros and test them with other sources. In the mean time, I'll answer any questions that anyone might have. Cheers!