UPDATE - BUYER BEWARE: I've owned these headphones for about seven months, and the headband just snapped FOR THE SECOND TIME. The headband broke the first time after about 4 months of regular use and I treat my headphones with care. Mailed them back and Beyer repaired them for free. However, they just snapped again today and it's only been three months since the last repair. I'm 100% convinced this is a design flaw, because they snapped at the same point both times, and the plastic is quite thin at the location of the break. I've emailed Beyer to see if they'll give me a credit toward a different model, because clearly these have a critical flaw. It's disappointing, because the sound and comfort of the DTX 350p is very good.
 
Original Review:
 
Design/Build Quality: I really like the minimal, all-black design of these cans. The logos are small and not in your face. The plastic frame is lightweight, but doesn't feel flimsy. The build feels fairly solid overall. Headband adjusts easily and has a thin metal support band inside. All the hinges and swivels feel solid, not loose or creaky. I like the flat "no-twist" cables, although a single-sided connection point would have been better. There are some mild microphonics when the cable brushes against my clothing, but pretty minimal and muffled. I have no interest in using these for working out, so it's pretty much a non-issue (I don't really notice it when I'm at my desk). However, the cable length is at least six inches too short, imo. I bought an extension to use these with my desktop (which sits on the floor beneath my desk). If you're using these with a phone or DAP in your front pocket, you'll probably be okay.

Comfort: They felt a tad snug out of the box, but I stretched them overnight, and they felt pretty good after that. The clamping force is not overbearing, yet they feel stable on my head. The earcups are a good size and provide a good amount of padding. The apex of the headband is also nicely padded. I can wear these for six or more hours sitting at my desk without any problems, and I have a fairly large head. The swivels ensure that the cups rest flatly against the ears. Again, these are very lightweight.

Sound: I'm not a hardcore audio guy, so I can't go into tremendous depth on the nuances here, but I'll do my best. My other cans are ATH M50 over-ear, which have excellent sound (in my opinion). Obviously, on-ear phones are not going to be in the same league, but the overall sound is pretty solid on the DTX 350. Highs are fairly crisp, although a little harsh at higher volumes. Mids are good. Bass has some punch but isn't overbearing. I believe the sound on these is actually fairly well-balanced. However, these headphones seem to get harsher at higher volumes. I can listen to music pretty loud on my M50s, but with these I find myself wanting to keep it lower. However, this may have more to do with the on-ear design than the actual sound quality, but I'm not sure. Obviously these offer very little isolation and there is sound bleed, but the trade-off is lightweight portability.

Final Thoughts: If you're looking for "hi-fi sound on a budget," I would invest a little more money on something better, such as ATH M50 or Sennheiser Momentum over-ear. But if you're looking for a small, lightweight pair of on-ear cans for work, commuting or travel, these are a solid choice.

I would like to preface this review by saying that I'm sort of an audio noob. I'm a music nut, but have only recently really started to get into the tech, and these are the first headphones I have purchased that cost more over $30. Man was I missing out!

What I listen to: Rock, Metal, Alternative, Indie, Post Rock, Folk, Classical, Techno, House, Dub Step, Ambient, Trip Hop, Hip-Hop... basically everything but country.

Audio quality: I read a lot of reviews slinging around esoteric jargon like "soundstage" and whatnot. I don't really have the audiophile vocab to describe every facet of the sound quality in detail. However, I will say that I've been pleased with my 448s so far. The mids and overall sound is very good. I've only burned them in for around 15-16 hours so far, so they probably aren't fully broken in yet. Admittedly, they sounded pretty good out of the box. I'm hoping the bass will become a little more punchy, but so far that is the only thing I've been disappointed with. The bass isn't terrible, it just doesn't quite have the fullness I was hoping for. However, according to the above reviewer, I need to keep burning them in.
 
Update: I received my Electric Avenues PA2V2 portable amp, but have not noticed a "significant" change in the audio quality or bass. However, I still have some burn-in to do on the Senns and the amp. Also, I still need to tweak the gain settings on the amp. I'll update again when I have it dialed in more.

Comfort: My only gripe about the comfort is that if the earcups were any smaller, they probably wouldn't fit (and I have fairly average sized ears, imo). As it is, they are fairly snug around my ears, with very little room to spare. Perhaps this is desirable for some people, but I feel like one or two more mm in the inner circumference would have been nice. (If you have Will Smith ears, these might not be the best for you). The faux leather is not great. I encounter some sweating when in a warm environment, so I have to wipe them off occasionally. I may look into some alternative earcups (velour perhaps), if I can find some. I will say that the 448s are nice and lightweight. They are also not too tight on the skull, which is a bonus.

Other likes/dislikes: These aren't the best looking headphones I've ever seen, but image is of least concern for me. These do cut down on external sound somewhat, although don't expect them to muffle screaming babies or anything. I appreciate the included cord extender. They also included a storage bag, which I haven't really used, but is still a nice addition. I'll try to update this review after more burn-in and when I get my amp adjusted.

Summary: I would say these are excellent for the price. For only $100, Senn 448s are a great first set of cans.