Pros: Superior Speed, Mids, and Bass
Cons: Expensive to Amp, Seal can be tricky
Headphones simply do not get better than this. The first time I heard the O2 mk1 was at the First National Meet. I was blown away. I had no idea that headphones could sound like...that. They simply melted away. It some ways, they ruined that meet for me because they were among the first phones I heard.
Fast forward a few years and a few meets later and I decided that I had committed to enough money to mediocrity, and I finally pulled the trigger on a pair on the used market. Amped with a KGSS DX, they are a splendid mixture of power and finesse. "Liquidy" is a word I associate with them. And there is not a single thing I would change about the sound signature. They are, simply put, the best headphones with which I have spent a meaningful amount of time.
The O2 mk1 is demanding, though. You will need to invest in a 'stat amp of some quality and feed it a decent source. But that investment will yield sonic bliss.