Build Quality/Comfort: Good build quality. The leatherette headband is soft and comfortable, the velour pads feel like velvet and are extremely comfortable. I can wear these all day with no discomfort. The head-band provides lots of extension to fit from the smallest to largest of heads. The fake wood really adds a nice touch to the look, though some may find these look over the top. The headphones are made of almost complete plastic, not cheap feeling McDonald's quality plastic but I expected better materials for the $200+ price tag.
Lows: Some may say it has little bass, but I think it hits the sweet spot. More bass and I could not listen to Jazz/Rock, less bass and I couldn't listen to Rap/Electronic. Sennheiser has done well here. The problem with the bass is it lacks subsonic power that some songs need to have to be enjoyable, it extends low but is polite with its presentation. This can be fixed with an equalizer (20-50hz +2-3db). No muddiness at all and is very tight sounding
Mids: Great vocals, the midrange is buttery smooth. The vocals are very forward sounding, alive, and pleasing.
Highs: Not very dark like many Sennheiser headphones. They actually have a bit of sparkle and can be fun sounding without being over bright and fatiguing like most Grado's. I have found little sibilance and the treble is quite enjoyable. They never seem to make music to dark or bright, drums almost never sound metallic and sharp.
Soundstage: Pretty big with good detail and layering. While not as huge as the AD-700, I find it easily makes up for it in quality. The soundstage is bigger on the HD598 than the HD600 but the HD600 has better quality. Instruments are layered beautifully, these never sound claustrophobic and closed.
Etc: I find they don't need a amp to perform well. Even though these have a impedance spike @ 300ohms my iPod with a line out dock drives these loud and clear. I notice the only thing that improves is bass performance and impact with an amp.
Final Thoughts: I highly recommend these, I doubt you will be disappointed unless you listen to very heavy bass/fast paced genres. The 598's are flat and pretty neutral without being overly dark or bright. An exceptional value at it's price!
EDIT: I did some further testing with some bass frequency sweeps, the headphones had much more bass quantity and impact with an amp. The impedance spike is a bit of a problem without an amp.