Pros: cheap, comfortable for 24/7, decent audio quality
Cons: from reviews I read, audio quality could be better
I will not write some looong review. Just things I noticed with my HD 555.
They need pretty long burn-in. But its worth it. You need decent audio source to get some quality from them. Headphone amp is not mandatory, but definetly helps. If you want, you can use iPod with them, it sounds pretty good, but it will eat your battery pretty fast. They are also suitable for using with PC. Which means they are fine with soundcards like ASUS Xonar STX or Auzentech X-Fi Forte 7.1 and Bravura 7.1.
Audio characteristic is kinda suprising, cause they are very neutral (at least to my ears). I had HD 650, which are far from being neutral (but they are fun) and AKG 701, which has those nice high pitched sound that will remove your brain from skull if you use them too long (curable by proper amp).
Those have nothing of this "qualities". They simply sound like they should. Maybe bit more emphasis on bass, not exactly briliant mids, and I find highs quite nice (though, HD 650 actually have better ones, but they should considering price).
Only real downside is build quality, after year they tend to crack where headband is attached to driver part. It will eventualy crack completely and fell apart. Its fixable, but it shouldnt be this way.
One of main "pros" is that they are comfortable. You can wear them for hours, even half day and you wont even notice them. I think that only HD 800 are such comfortable things (probably only way you can compare them to HD 800 :D ).
Thats all, if you want Sennheiser for some all-around activity (I suggest listening music on PC, iPod, or gaming) its good choice. HD 595 are similar, just have bit "crisper/clearer" sound.
Yea and if you want to game on PC and use 3D sound effects, you should buy ASUS soundcard, as Creative CMSS 3D doesnt work too much with this (it works best with closed cans, not open ones).
PS: "Sound reflector" is piece of tape-foam inside headphones, google out how to remove it and do it. :) (its easy and safe as long as you dont drop anything inside drivers, metal screws for example are very bad idea). If you leave that foam there, you will be tired fast from listening and sound clarity will be much worse than it should be. But please, be careful while doing this "mod" and dont destroy your headphones.