The K540 feels pretty sturdy in the hands. The adjustable fit feels a little clickety and plastic-like, and I'm a little spoiled by the self-ajusting bands of the higher AKG models, but the fit is nice and comfy, and the padding is some kind of velour if I'm not mistaken.
As for the sound, it's a rather bright set of cans. The mids are pretty forward and the highs are a bit on the edgy side. Since they're semi open, there's also a sense of space, but not as wide as say the 242HDs. The lows felt a bit flat for me, and they lacked a sense of warmth. Very good for rock tracks as far as I tried them out, but they did well on vocal tracks as well. I couldn't find myself thumping to the beat of some trance I had lying around in my iPod, but it's still a pretty dynamic set of headphones - at least from first impressions.
It looks like it can be used without an amp, but I bet they could use some power from a decent portable amp, like an E5 maybe.