Pros: Embraces all music genres, super comfortable, not so expensive.
Cons: After 6 months of use, still haven't found any.
First of all, I still didn't have the opportunity of using an HD 598, which is the 500 series flagship. So I can only compare what I know about the 518 with what people talk about the 598.
Many reviewers say the HD 598 has a great soundstage with beautifully fluid highs and mids, but lacks punch in bass. A lot of users say these same things about it, so I assume these characteristics are true. The 598 was my first option, but sincerely I think those body colors are ugly, and they still cost too much for my possibilities. So, when I saw the 518, with the same appearance but nice design/colors and promise of almost same sound quality without spending too much, I didn't think twice.
It's hard to explain how awesome these headphones are. It's hard to believe how well he behaves with ALL music styles (I'm a music lover, I listen to almost every genre), and boy, he really delivers the bass, opposing what people talk about the 598. I saw in another website that his sound signature was kind of "tuned" for bass, that makes him different from the 558 and 598. Probably lost a bit of soundstage in exchange of it.
Hip-hop and electronica? The beats will punch your face. Classical? Aerial soundstage, flawless separation of instruments. Rock/heavy metal? High volumes, no distortion. You can listen to the bass lines even in the most chaotic death metal. Jazz? I'll just say this: if you get the 518, listen to Dave Brubeck's "Take Five". You'll understand. And all this with default EQ settings and no amp. Make some EQ fine-tuning plugged in an headphone amplifier and these headphones will definitely blow your mind.
Extremely comfortable, thick wires that can be easily replaced, beautiful grilled design. All this for less than $150. This is really precious, Sennheiser impresses me more at each day. These headphones are the ultimate definition of "bang-for-the-buck".