I bought my pair of DT231's in a sale while looking for some decent-sounding headphones at a sensible (consumer, not audiophile) price. I knew about the solid reputation Beyerdyamic has in high-end phones, but couldn't find much on any of their cheaper models. So the purchase was a bit of a gamble, but fortunately it turned out well overall!
The build quality of these is really good considering the price. The look and feel gives you the impression that these are a step up from cheaper, hi-fi headphones commonly produced most major hi-fi brands. The cable is completely rounded and flexible, so it tangles a lot less easily. The plastic used in the headband is nice and flexible although I'm not sure it would stand up to regular abuse (e.g. from DJing).
The comfort is also good. When you put these headphones on you know they're there but it's not an unpleasant feeling at all. I found the pressure they exert to be light and the velour cushions don't irritate your skin anywhere near as much as the "faux leather" often supplied with other brands. The ear cups are quite small though and don't surround my entire ears. While I still found them perfectly comfortable, this may be an issue for some people. The noise isolation, perhaps as a result of this, is acceptable but not great.
So all that's left to talk about is the sound quality.
I found them loud enough to use with an iPod, as long as the volume was set fairly high, around 60-90%. My first impressions of the sound was a clarity and openness that really brought out all of the detail in my music, particularly at high frequencies. They are the total opposite of "muffled". In fact, when I first started using these, I felt the treble was too much for my own taste, so if you're really not a fan of strong treble, you might want to avoid these at least until you've tried them on first. I soon got used to it though.
The low frequencies unfortunately are a bit weak for many modern tracks. The bass is definitely there, a million times more than on standard ipod earbuds for example, and it's nice and tight rather than a tiring boomy sound. It's just still a bit quiet. I often found myself turning up the bass boost dial on my amp. Obviously this is down to the types of music you listen to and personal preference, but as much as I otherwise like them, I couldn't recommend these headphones to someone specifically looking for bass.
For most people's general listening though, I believe they are good sounding for money and certainly worth a lot of consideration.