I just got a set of these headphones to replace my last pair and so far I'm more than happy with the results. Unlike my last pair, these Sennheisers really keep the sound in so no more complaints of music leakage from my office mates! I was able to turn up way loud without anyone hearing anything.
It's worth remembering that these are not audiophile headphones, which is to say that they don't have the best dynamic range or the rich treble and bass of some headphones. What they do have is a great reference sound with excellent clarity and separation, and a bit of mid-range bias, which will be fine once your ears get used to them.
Comfort is subjective. Personally I like the way they clamp strongly onto my ears but it does make them feel less lightweight than they really are. Build quality is great, they seem very robust, with a good long cable, plus they come with a storage bag. Overall I can't imagine getting better sound quality or value for the money.