I've had mine for about a year now. I bought them based on reports of being very balanced and suitable for many kinds of music. Hearing them in a Hi-Fi store, I was immediately swept by what must by Sennheiser's signature 'speaker-like' sound. I found the same effect on the 555's, but thought I may as well pay the extra and have a more refined one for amping.
I love them, they have a great mix of clarity and bass, with a very pleasing musicality, which makes them so enjoyable. My only other dynamic cans are AKG 240s, which have no-where near the clarity, but their comparative 'roughness', bass impact and a wider sound-stage makes them better for action movies or grungy-sounding rock music. For anything else, though, I am wearing the HD 595s, which also amaze listening friends, who have often only ever tried really low-end headphones and are surprised at what is possible, even at this 'mid' level.
I always listen to them amped now. It's true that in terms of volume they can go unamped (I have the newer 50ohms version), but the sound is thin and the bass is pretty flabby by comparison. I'd rather just listen to my inner-ear buds this way. With the amping, they tighten up and sound a lot more satisfying. Still, they sound good unamped and that's one reason I got them, as I wanted something that could be used in all kinds of situations. Based on earlier comments, I'd say it is very possible that the 555's are actually better unamped- at least in the store they sounded satisfying enough. The question is- would you really want both? The bass is a little light for movies, so I listen to them with my sub-woffer on, which works really well, turning them into a pair of high-class satellite speakers right on my ears!
Next stop- the HD 600 for classical symphonies, which sound a little too close on the 595's. My conclusion now is that they are a great phone, but despite being a good all-rounder, there will always be phones that excel in certain areas more.
