Got my Sin City BD yesterday, (it arrived in 1 day from Amazon!).
Apparently there's about 20 minutes of extra material on the extended cut version. I just watched a few parts and it looks pretty good, but not a very big quality difference from the DVD.
One bonus though, it came with a $10 rebate coupon inside. Just clip the proof of purchase off the DVD and the BD and mail 'em in, and they send you $10!! I bought the BD regardless, but with the $10 rebate, the BD only ends up costing $14! Pretty sweet.
Jessica Alba in leather chaps in high def. Yyuuuummmmmmm.