The HD650 is Sennheiser's currently flagship headphone.
The sound quality it outstanding. The character of the headphones is relatively neutral, perhaps a bit on the dark side. If you like a very bright headphone then I would recommend a Grado or Beyerdynamic set. The highs are very detailed without being too bright, which is what I love about them. The bass reponse is phenominal as well; nothing short of expensive floorstanding speakers are going to deliver the right combination of punch and volume that these headphones can.
I will say, however, that if you're listening to music without a headphone amplifier, your money is best spent elsewhere. While you can use the HD650s to listen to your ipod, their potential is being severely limited. I didn't believe this at first and was mildly disappointed with my headphones. After I purchased an amp, there was a staggering improvement in sound.