The optical performance difference between the two lenses looks to be quite significant if you go by the MTF chart comparison.
I had two complaints about the original 35L, the first being it wasn't weather sealed and the second was the CA, it looks like both of those issues have been corrected. I'm looking forward to seeing if the additional weight is going to be difficult to get used to with the new lens, I did like the compact size of the first version.
Unfortunately, there aren't many sample images from the new lens yet, but they should start making their way to the web soon.
Preorder EF 35 f/1.4L II $1799: B&H Photo | Adorama | Amazon