Hi
I suppose it all depends what music type you intend to listen to, what your personal preference is in terms of the "presentation" of the sound.
I have the HD595's. They are probably one of the most comfortable headphones and the build quality is excellent as well.
With 50 ohms impedance, they are easy to drive from your mp3 player or laptop without getting a headphone amp.
The laid back non-fatiguing sound, good soundstaging and adeqaute bass makes it a good " all-rounder"headphone. Anyway, there are plenty of reviews and descriptions on head-fi for you to check out at length
I would second the suggestion on the Beyerdynamic DT-880, which is a very good heaphone with a different character to the Sennheiser.There is a low impedance version of this model also. I'm sure the A700 you are considering is very good as well, but I havent tried that myself yet.
Hope that helps
Cheers