Pros: Very comfortable to wear (if the plastic headband has decent flex), very nicely balanced sound that's non-fatiguing
Cons: You do have to replace the earpads from time to time; open-air design does not work for all situations
I own two pair of these, both the original 600-ohm model from the mid 70s. I still use them quite frequently, they are my favorite headphone.
One pair has a tighter headband with less flex - probably my "2nd pair" that did not get used as much. This squeezes my head a bit - and I have a smallish head as heads go. So if you are buying a used pair, be sure to check this aspect for your comfort. Replacement parts are harder to come by now.
There are still a variety of cord replacements available with the right earpiece jacks, the newer shorter cords with the mini-plugs work well. I found a replacement cord for around $12 shipped that I like better than the OEM Sennheiser cords for $30.
The main concern I hear expressed about these is the impedance. With my convenient sources such as cell phones and iPods, the volume level is high enough for me for normal listening. You can't get it super loud, but that's very bad for ears anyway. With a headphone amp such as the Schiit Fulla, or a normal headphone in on a receiver, there is of course no problem at all.
After 40+ years of continuous use, the drivers are still just fine. The plastic they used, don't know what it is, but mine have not yellowed at all. I suppose if you use outdoors a lot, that might be different.
If you like open-air designs, these are worth tracking down. To me, they are "THE" headphone, because they were my first. Even that awful original box photo makes me smile, because I remember buying them.
My hearing now extends only to about 13,600cps so these have quite sufficient highs for me. I suppose some young ears might deem the top end a bit rolled off. Bass is good for me, not bloated, just right.