Pros: Big step up from HD 515, Mids and Highs are clear and vivid, Bass is more pronounced but still laid back, Comfortable for long listening sessions.
Cons: Plastic material (still feels and looks good though)
Design - The louvered design is something people will either like or hate. I personally like it since it gives it a very engineered look, just like a German car. The color combination is stylish and gives it a high end feel, very nice use of graphite and black with some silver highlights. I personally prefer these colors versus the 598.
Comfort - The velour pads have an edge in the inside but are still very comfortable, but the 558 pads are still a little bit better, they're rounded on the inside edges and are not as 'springy'. I can still listen to these cans for hours on end though. I also wanted to add that one of the reasons (I assume, aside from making it more affordable) Sennheiser chose to use plastic versus metal, is because it has less weight and therefore has less stress to the head and neck resulting in hours of listening pleasure.
Audio Quality - The mids and highs are clear, the sound stage is excellent! You can actually hear the little nuances in the different instruments being played (Have a listen to the Cure's - A Forest, the acoustic version or something similar) and where they are positioned. In terms of bass these cans actually have good bass, albeit more on the laid back side (also one of the reasons these headphones are not tiring to the ears when listening for hours) when played without an amp. Speaking of amps, when used with one it starts to really shine. I used my fiio E11 and the overall sound becomes warmer and tighter, the bass becomes fuller and deeper. Remember this still has the Sennheiser signature 'veiled' sound but with more presence.
Value - For it's current price of $120 I think it's a winner, though if you could get these for somewhere around the $100 mark you're golden! It sounds fantastic and with the sound quality you won't feel like you over paid (unless you're a bass head that just wants deafening lows). Another plus with these cans is it can be powered by a portable amp with no issues like the Fiio E11. Once paired, you can't go wrong! Remember though, even without one as long as you setup your EQ properly it still sounds excellent (not for bass heads though)!
EDIT (Jan 6, 2013 update): After 100+ Hours of burning in these headphones, they have definitely improved in the low frequency area making the the bass sound tighter. This applies to people who use no amps at all with these headphones.
For people who use amps, please note that there can be a difference how the low end is delivered. My example would come from my experience with the Fiio E11 versus the ZO2. I personally prefer the ZO2 with these headphones since the bass is definitely tighter and it helps the headphones literally cover all music genres I listen to (alternative, rock, classic rock, rap, hip-hop, r&b, punk, dub step, electronica, classical, some pop artists). For the comparison between amplifiers it's broken down into the two below:
Darker and warm overall while giving the HD-518 oomph in the lower frequencies
When listening to these types of music: Jazz, Pop, and Classical it is perfect and no muddling happens. The bass presented is tight and responds well. It has a tendency to be muddled when listening to R&B (Mary J. Blige), Rap (Dr. Dre and Snoop) and Linkin Park.
Still presented very well and retains the HD-518 personality here, which is mid forward.
Still presented very well but slightly recessed and retains the HD-518 personality.
Overall the sound stage is retained with very little loss in placement.
It helps retain the overall Sennheiser signature while givng it some low end oomph. Awesome for Classical, Pop, Rock, Classic Rock, Metal, Vocals and some Alternative music.
It muddles the bass when listening to R&B, Rap and some Linkin Park and the likes.
Warmer and fun overall making the HD518 shine in all types of music
No muddling of Bass happens here, the issue I had with the E11 when listening to Mary J. Blige, Dr. Dre , Snoop and even Linkin Park disappeared with this amp!
Bass response was quicker so the Linkin Park songs sounded nicer when the lower frequencies were presented. Bass oomph I wanted when listening to R&B and Rap were tighter and faster with no muddling, making Mary J. Blige, Dr. Dre and Snoop music when low frequencies were presented sound outstanding!
Still presented very well and retains the mid forward Sennhesier signature.
Still presented well and slightly recessed.
Overall the sound stage is still there but not as open compared to the Fiio E11.
It makes the HD-518 a perfect all rounder. With this combination I can definitely listen to any type of music genre and be happy with how the High, Mid and Low sound. Bass is tighter and no muddling happens! Makes the HD-518 fun cans to listen to!
It slightly changes the sound of the HD-518 reducing the veil sound signature of the Sennheiser. Not powerful enough (volume is limited when using LOD) to drive HD-518 and higher impedance cans when using LOD. You need to use the headphone output to drive to make the headphone and the amp shine.
NOTE: These cans (as with most 500/600 series HP) tend to highlight poorly recorded or low bit rate recordings. You will hear the imperfections in the track if you don't use flac.
Overall rating of 4.5 out of 5 stars