I've owned the 598s for a year now, and they were one of my favorite headphones up to buying the hd 600s. The 598s have a wider soundstage & more comfortable fit/clamp, but there are quite a few areas in which they can't compete with the 600s. For instance, the 600s are more balanced, so you can listen at higher volumes without feeling as if the mids are over-presented. A lot of people say the 598s are brighter, but I disagree heavily. Overall, the 600s are more lush & detailed without being fatiguing. Some would say more analytical. It's actually one of the captivating aspects of them as I'm noticing more details in my music now. The timbre on the 600s just sound more accurate & refined. You'll notice this, for instance, when you compare violins on both cans. There's slightly punchier bass on the 600s & while the mids aren't as prominent as with the 598s, I think they can be quite intimate and plentiful. They're more neutral. There's also better instrument separation & the soundstage is actually quite good.
Both cans do not fatigue my ears with high frequencies. I'm particularly sensitive to that aspect which is why I've had to return the ATH-m50s, AKG K 240 mKII, & the AKG K 612s. If you aren't sensitive to a brighter headphone, I would say the 612s are actually quite good, especially for the price. The other issue I had with them was their large earpads, as they are dependent very much on positioning and I think they feel a bit awkward with all of the extra space. Sennheiser 598/600 pads make the most sense to me.
The 598s are great with both movies & games. That's what I primarily use them for as they are accommodating with glasses. I don't use the 600s with glasses since I don't like how indented the pads get since there's a stronger clamp. I just use them for music, close my eyes, and get lost in the music.