From what I've read, it's unwise to consider the MDR-V6 even though it is similarly priced to the MDR-7506. If you are considering one of those two headphones, go with the latter.
Biggiesized - I suspect you're thinking of the V600 rather than the V6. I haven't listened to the V600 myself, but have seen many posted cautions to make sure you're getting the V6 and NOT the V600.
Over on the HeadFi forums there have been several threads over the last couple of years re whether there IS any difference between the V6 and the 7506 - other than price (7506=$100, V6=$80) and the color of a couple of plastic trim pieces. Some claim to be able to hear a difference, others say thats rubbish. There doesn't seem to be any physical difference between them, but there is some speculation that Sony hand-picks the drivers for the 7506.
Re the sound of the V6, while many posters love them, some have complained they are too "clinical", maybe a little too bright, and somewhat lacking in bass. Mine don't sound that way to me. The highs certainly don't sound overly bright (although that could be because at my age I'm probably missing a few db of sensitivity in the top octave). As for them sounding clinical, I just think of them as neutral and very detailed - that just happens to be the sound I like. Regarding the bass, they will get WAY down there, but to my ears do so in a very controlled fashion; avaserfi was right when he described the base as "tight." If you like boomy, in-your-face bass, they may not be the cans for you.
For the $400 landonspop mentioned, I'm sure you can find better. But for anybody who likes neutral sound and only has a $100 to spend, they will be hard pressed to find anything more satisfying than either the 7506 or the V6.
Cheers