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 $100 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.
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
-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.
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).