Sounds like an amazing headphone. I might just have to splurge and buy it. I would love a comparison between these and the D2000
The bass is unlike anything I've ever heard in headphones, it really goes all the way down in the frequency. I would say its not bloated or overemphasized, its tight and it makes its presence felt. I say its not overemphasized because it doesn't make you tired of listening. They are very comfortable. I cannot compare them to the XB 500/700, so I cannot say if it is worth the extra $$, but if you want to get the best in bass, you probably want this.
What I like most about these headphones is that unlike most bassy headphones the highs aren't too pronounced. My DT 990 600 ohms had great bass, but I could hardly turn the volume up or listen for extended periods of time because the high frequencies would bother my ears (keep in mind I am 21 years old, I think people who don't notice this or aren't bothered by it are usually older). These make you want to turn the volume up. WAY up. They sound great loud (E7 + E9) but as others have mentioned, you have to keep in mind you might be hurting your ears if you keep the volume up for a long time. However, it really does feel like you are in the middle of a professionally tuned dance floor (like DJ amtrax said) when you're "inside" of these headphones. They are so big and sound so good loud that it really feels like you are in there.
Amazing for electronica, superb for dubstep. Rock can sound pretty good cause you really feel the drums kick, but obviously if you're considering these headphones this can't be the genre you are most concerned about. It seems to be like the vocals are a bit recessed, but I don't think any headphones focused on bass are different. Besides its not bad enough that it takes away from the music (Regina Spektor still sounds great). It just takes the attention away from the singer and drives it towards the drums and bass guitar.
I would say that if you like bass but you don't like headphones that sound too sharp, this choice is perfect for you, especially if electronica is your thing.