RockStar2005
Headphoneus Supremus
While I haven't tried the DT990s, from what I've heard and read, they are really bassy, whereas the HD598 aim for a more balanced response, so in comparison, it should be that way - the HD598 most probably have less bass than the 990s. That doesn't mean they lack bass, though. On the contrary, lows are present when they are called for, and the headphone even has a warm sound signature to it.
However, there seems to be a consensus that the 990s are considered higher end stuff than the 598s. So there should be an overall drop in sound quality. Where? How much? I don't know, I haven't tried the 990. But as for me, as a happy 598 listener, I think at that price, they're great for what they are - a budget hi-fi headphone. I don't think you'll find anything better under $200.
All in all, I will always recommend the 598s, especially at the discounted $100-130 price. So, back to your question, 'are they worth the price?', I'd say, 'yeah, totally!'. Go for them.
I agree with Veggie, airman. I've tried out both the 598 and the 598 SE, and they do sound GREAT! With regards to comfort, I initially found the 598 uncomfortable, but a year or so later I tried the 598 SE, and I found that if I put them on, then angle the earcups so the top parts point INWARD at me, they become much more comfortable.
I also suggest you read my review of the 598 SE vs. my AKG K553 Pro (which I just replaced with the K550 MK II, same sound signature though). You may find it helpful. But yes, that price is MONEY!