Pros: Love them over my HE-5/EF-5, supremely comfortable, nigh indestructible, not great, but solid sound quality for the price (when foam modded)
Cons: Other headphones around the same price probably perform better (depending on your uses) with better SQ, isolation, improve more when amped etc.
I personally wouldn't buy these again now (that I know more about audio) than $75 new. However, these are my first headphones, and I have some sentimental attachments to them. I actually prefer these to my HE-5/EF-5 currently, though unfortunately it is down and I can't do a perfect A/B comparison for now.
I wouldn't recommend anyone to get these now, but these were the first phones I toted to school and convinced three of my friends to purchase decent audio. All of them have gotten Senns (two of them with the HD555s - one used, the other I will sell mine to, the other with a very similar PC 350, which is probably not any better, unfortunately). I do hope that they will also eventually move on and see even further up the chain, not necessarily relegated to Sennheiser's offerings (not an insult to these, I love them).
SQ wise, they do everything I need them to. Easy to drive, without fatiguing bass, though I'd prefer to have some more isolation for portable listening (will try ES-10s as the sole HP for both home and portable, though there is a good chance that may not work too).
In a nutshell, they have good SQ for the price, but they aren't the creme of the crop any more at the price range. However, I've had extremely positive experiences with comfort and build quality. These are supremely comfortable and I don't even notice them on much of the time. I leave them always even when I'm not playing anything. The velvety-like pads are also great for a slight bit of insulation on a cold day (though on the flipside, they are bad in heat over 28-30C, though that is infrequent in Vancouver, Canada).
They also appear to be indestructible to me. Throw in a few hundred hours of moderate to extreme rain, much use both indoors and out, being dropped several dozens of times, slammed, dropped from relatively high perches, hitting concrete and other hard surfaces, having its gold plated connector bent out of shape half a dozen times, and still working strong - that is simply incredible. Not everyone has had such a positive experience in this department, but I absolutely adore them for this and they will always hold sentimental value to me. I hope the future owner of my pair of HD 555s treats and enjoys them well.
Edit: For the purchase date, I believe I bought it in May 2008, give or take a month. Also edited ratings a bit to be a bit more fair.
Edit 2: A lot of owners complain about issues with build quality, which is another reason why I would never suggest anyone to buy these phones now, with better options for whatever use you may have for these unless you can get these for under $100, preferably in the 70s range. However, the unsatisfied are always the more vocal bunch, and this has been perfect for me in my 2 years of extreme abuse (no kidding).
Edit 3: A (possibly) final append to my review - despite the low impedance and high sensitivity, these are NOT the definition of easy to drive. Yes, they are very easy to drive at home, and both my Auzentech X-Fi Prelude & Nuforce uDac will drive them to their potential, but they will NOT sound good straight out of an iPod or DAP (in my case, a Cowon S9) [Actually, the uDac does a much better job, as the Prelude botches the bass, has worse separation and sounds more veiled and muddy]. Look for phones made specifically for portable in this case - a 3 M cable, a 1/4" to 1/8" adapter plus sub-optimal sound quality overall is atrocious for outside usage, not to mention excessive sound bleeding and nearly no isolation (they're open cans, after all). ES-10s, in contrast, are excellent even when driven straight out of a DAP with no external (even portable) amplification or DAP, and I actually prefer the sound signature out of my S9 over my uDac.
ES10s of course, also do benefit greatly from superior amplification.