I did a bunch of reseach on head-fi.org before I bought mine, and the consensus there was the 700DJ's were crap, but the V6 and 7506 were highly recommended. I would check out and browse the message boards. HTH!
I have a pair of the Sennheiser HD570 headphones... I believe Amazon has them for around $85, which is a steal compared to the ~$130 you see them elsewhere. They are real nice - very lightweight and comfortable, and they have a great sound. Besides Sennheiser, I also liked the sound of AKG headphones (however I felt the heavy design would be more prone to giving me headphone fatigue). Drive to your nearest Guitar Center or similar store and see if you can demo some pairs.
One other thing, make sure you test the headphones on sources similar to what you'll be using them with... while I'm amazed by the sound from the Senns when hooked up to my receiver, they suffer a bit when driven from my PC or a portable mp3 player. I've bought necessary parts to build a headphone amp for use with PC, but I haven't gotten around to building it yet.
The DJ suffix means the headphones are intended for DJ use in a club environment. While they are not the most accurate sounding headphones, they are quite robust and handle bass very well. I'm not a big fan of headphones for home use but do use a pair of the 700s when I'm on the road. Surprisingly, almost any type of laptop, MP3, CD, or Minidisc player can power these headphones to very loud volumes. They work just as well as noise-cancelling headphones on airplane trips because they seal around the ears very well and let very little sound leak out to bother neighboring passengers. Mine are comfortable for long periods of use. Although the headphones are large, they fold to be quite small. I'd recommend them for video game and use away from home, but would suggest a more neutral sounding headphone for home use or for monitoring recordings.
I have owned the V6s and have heard the DJs, I agree with those above that the V6/7506 are excellent headphones but I also really like the sound the DJs produce, if you drove them with a small amp I bet they would be incredible sounding. The only problem with the VJs that I found and the only reason I dont have a pair is because they are uncomfortable. I'd call the VJs a fun sounding headphone, you will either hate them or like them depending on what your tastes are.
The V6s are easy to drive so they sound pretty good with anything, they are comfortable and have a nice sonic signature that suites alot of people hence all the good reviews.