I am so impressed by these headphones and the more they break-in, the better they sound. I would call myself an audiophile (I'm very picky over audio gear) and by every measure - these are not only excellent value for the money, but for many, many times the price!
For comparison, I also own the Shure SE530 IEM's and I prefer the Philips headphones, hands down. The SE530's have a dark, 'cupped' sound (in the midrange) and lack real bass impact. The SE530's also have strange, rolled off highs above 17Khz or so.
The 3580's sound open, sweet, clean and they have some of the best bass reproduction I ever heard from a IEM. It's as deep as the ocean with beautiful impact, definition and doesn't muddy up the midbass. The midrange is well defined with excellent clarity. The highs are clean with a tad bit of sweetness with a slight roll off that compliments the rest of the spectrum. At first, the highs sound a little aggressive, but that quickly burns off after a few hours. If I have one complaint, it would be that I would like a bit more airiness to the highs, but that's really nit-picking. As for comfort, they are so light I can almost forget I'm wearing them. With a clean and clear frequency spectrum, I found myself listening to them for hours on end with no fatigue or pain as opposed to the SE530's which give me a headache and earache over long listening sessions.
Getting a good fit and seal is crucial to get the best sound quality out of any IEM and the 3580's are no exception. If you get a pair and it sounds thin, try changing the silicone tips. I get the best sound with the medium size tips.
I'm having a lot of fun with these headphones. I find myself bobbing my head and tapping my foot the more I listen to them. After more break-in, I'll write a more thorough review. So far, I'm very impressed.