Pros: Extremely comfortable (near weightless), excellent mid range and large soundstage
Cons: Build quality is a little bit flimsy, bass is light and sound is a little lean.
Even though I have several cans that do many things better than the K601, I still found this to be my go-to headphone for some laid back listening before and after work. AKG engineered a winner in regards to comfort and engaging mid-fi sound reproduction that just does not get fatiguing.
This headphone is all about the mids! Not particularly detailed and therefore fairly forgiving, but very involving and musical. Bass is light, but packs a small punch and one can appreciate that it goes deep. Highs are a apparent although a little rolled off on the very top end, but this helps it keep its laid back signature.
Not balanced in the sense of reproduction, but very balanced in the sense that it does almost every genre well. If you're looking to take a jump from $100 headphones to $200 headphones and are looking for a laid back headphone as described above, then these are your guys. Note, you'll want an amp with a bit of a punch as the K601 is power hungry, although note that matching synergy is important as I found them to sound better via my Travagan's Red amp (with AD743 opamps) rather than HiFiMAN EF5 which is far superior in the power category.
A brief review, but if you have any more questions, just ask!