OK I just bought myself a pair of MDR XB 700's 3 days ago to see for myself what all the fuss was about.
Overall so far I'm pleased with my purchase. Although I find these cans generally speaking a fail with metal/rock/grunge they completely own when they're used on the electronica/rap genres. These cans have some very nice deep bass slam and I found the bass to be generally well controlled and acceptably defined for my tastes. The highs have passable detail without being shrill and the mids are generally also good. I'm surprised how good the bass slam on this thing is. You get some very nice low frequency slam without it being bloated, muddy, and most importantly the bass doesn't bleed and overpower the mids and highs. The form factor is generally nice and I was glad to find out that a nice leather poach for storage is included when you buy the headset. Although these cans are mostly plastic they don't feel flimsy in my hands and feel like they will last a long time with proper care. The fit is excellent and the clamping force for my head is just right. There may be better headphones out there that can do the bass head thing better but considering most of the competition is pricier the XB 700's are a steal for the kind of sound signature your getting. These cans sound signature may not be prim proper snooty audiophile grade sound but they are a incredibly fun sound signature to groove to and play around with different electronica songs with.
For those who already have the XB 700 get the album Tweekend by The Crystal Method. This album sounds so freaking neat on these headphones. I couldn't help but smile while I listened to it on these cans for the first time.
I wish Sony had included an alternate product package with this model. I would gladly have thrown an extra 10 or 20 bucks down if I could get a decent protective box for this headset instead of the standard issue leather pouch. These cans also lack a 1/4 inch plug so if you want to hook up (depending on your gear) you'll need to have your own 1/4 inch jack on hand. Although these cans are being marketed as mobile headphones they are far from ideal as on the go cans.They're rather big and don't fold up so you'll need room in your bag for them and because they only come with a soft poach you'll have to be careful of hard knocks etc if you don't want to ruin them. These are not a good choice if you're seeking an all purpose do it all set of cans. If you like your industrial/metal/rock/grunge overly bass heavy then OK whatever floats your boat but for most people these headphones won't work for certain genres. Put simply these can's aren't a headbanging headphone they're cans meant for the bass head not the rocker. They're also not audiophile headphones so if you want to dissect music get a pair of mid/higher end AKG's, Sennheisers or Grado's. As said above in the Positives section if you just want a fun sound signature to groove to then these are the cans for you.
The LoL section:
When I first bought these cans I couldn't wait to get home and listen so I ripped open the packaging and plugged them into my iPod and wore them while doing my errands for the evening. In a span of two hours I had 1 guy say "nice headphones, must be nice to have headphones that double as pillows" and 2 friends who I ran across commented "hey princess Leia when did you get a sex change and become a man". Be prepared for a little light ribbing when wearing these in public especially if you have friends who are Star Wars geeks.