I didn't find any reviews of these headphones on here, so I figured I'd order a pair and attempt to give the community a sense of the sound signature of these. I paid about $100 for them from Razordogdeals on eBay. I think that with a bit of equalization, they are worth their selling price. I probably wouldn't pay retail for them, though ($180).
They are fairly comfortable, though not as comfortable as my HD 580s. This is for a few reasons: they clamp more, and the earpads are stiffer, in addition to having a bit of a ridge on the interior of the earpads (see picture), as opposed to the more curved, sloping design of the 580s' earpads. The velour on the 580s is nice, too. They're still quite comfortable, though, and can be worn for a few hours without too much unpleasantness. The drivers are angled, and there's plenty of room for my ears in the earcups.
Out of the box, they were fairly messy, sounding hollow and muddy (poor combination). But I'm starting to believe in burn-in. After a couple hours, they've tightened up a bit. These are still warm and bassy headphones. The impact is a bit wooly. In comparison with the Denon D1100s, I would say these are like lying in a bathtub of warm water, whereas the Denons are like lying in a bathtub of chocolate pudding inundated with large chunks of Hershey's bars. The bass warmth and impact isn't as powerful. The HD 518s are very inoffensive and non-fatiguing, with very little, if any, sibilance to be found. They are still quite forward and detailed, though, with good clarity. Those impressions are of the Sennheisers with no equalization. I tried a bit of equalization, and I've found the Acoustic setting on iTunes to be quite good. It seems to tighten things up more, while bringing more impact to the table, and they become more engaging and lively. I've got them running from my Fiio E9. I like the fact that the cable is a straight 1/4" (it comes with an adapter), and it's detachable, which is a nice touch. This review is a bit disjointed, but I wanted to give some impressions of their general sound signature. Feel free to ask questions, and I'll try and answer them as best I can. Pictures: