Pros: Forward, engaging mids. Overall euphonic, non-fatiguing, super comfortable, etc.
Cons: Lacking deep bass extension
At the time of this writing, these headphones cost less than $130 on Amazon. At that price point, they are an incredible value. For some reason, these headphones seem to be damned with faint praise in many reviews on the web. I think some of the professional headphone reviewers are afraid to give top marks to inexpensive headphones, so they use terms like "more refined" for the more expensive ones. I enjoyed listening to the HD558 more than my $400 HD600 (yes even with a good DAC/AMP), so I sold the HD600 and continue to enjoy the HD558. If that makes me unrefined, so be it!
What the 558 offer:
-Beautiful, natural, relatively neutral sound overall
-Forward, engaging mids (what I like best about these)
-Rich bass with good bass impact but not exagerrated bass (look elsewhere for "bass monsters")
-One-sided, replaceable cable (my favorite type)
-Good looks (my second favorite looking current-production Sennheisers after the HD650)
-Easy to drive (50 ohm impedance means you can drive these easily with iPod, phone, etc)
-Supremely comfortable on my fat head
What they don't offer (and neither did the HD600):
-Great sub-bass extension
Other points to consider
-As an open design, these do not offer noise blocking or prevent leakage (this is not a con, per se)
-They come with a 10ft (really long) straight cord terminating in a 6.35 mm stereo plug. The 3.5mm adapter is a bit unwieldy for use with phones, etc. However, Sennheiser offers a separately sold inexpensive 4.5ft replacement cable with a 3.5mm termination, perfect for such uses
If you are not using a dedicated amp and are looking to plug headphones directly into your phone/MP3 player/laptop/computer, don't need closed headphones to keep sound out/in, then IMO these are the best Sennheisers available and one of the best choices of any brand at any price point. Of course they sound even better with a dedicated amp. I'm primarily using them with a Schiit Modi DAC and Asgard 2 amp, and they sound fantastic.
The only limitation I find is that the bass extension is not adequate for 2% of my rap and electronic music. Eg, ODB's "Harlem World" or MF Doom's "Hey". Otherwise they sound fantastic with all genres (including rock, jazz, pop, and 98% of my rap/electronic).