I've had the DT 231's for about two years. They were my first real set of phones (well, I had Sony MDR-7506, which I hated, and the AKG 240's, which I didn't much like either).
I picked them up at the reccomendation of Tyll over at www.headphone.com.
They are a closed design, but they don't provide too much isolation from outside noise. What they do provide is isolation for the sounds comming out of the headphones, meaning the 20 people on the tube with you don't have to rock out to your Coldplay. They've been great on long plane/train rides.
Sound-wise, I think they're okay. They lack the air of open phones, and I can tell the difference between them and my Koss KC-35's. The highs on the Koss phones sound much better, but I think the overall sound of the Dt-231's is more cohesive and even. They aren't too big on the low-end, but provide decent, non-bloated bass. I feel the KC-35's can get a bit boomy and overbearing in their lows some times.
They are very comfortable phones to wear, with a good self-adjusting headband and plush pads. They're made from solid plastic, and mine have taken a beating over the years and held up just fine.
Overall, I'd say get them if having a closed phone is important, but not too important. But if you don't need a closed phone, think about the Koss KC-35's or the Porta Pros; both these little guys have some killer sound quality.