Summary: I got these on a killer bargain from Fry's late last month. After two weeks I can say that my initial impression of them has improved but, in short, you should get the 558s and do the foam tape mod instead of buying these.
Pros: Solid highs, earpads that breathe very well, removable cable
Cons: Boomy bass, lacking mids, dumbest cable ever, cups don't turn into the shape of a "V" enough, too closed
Summary: In the end though, the HD518s end up being more of a luxury buy at that price rather than the entry-level audiophile headphones they were intended to be. They perform well enough to fit rather comfortably into either category though, so Sennheiser definitely have released a pretty good product.