First of all, I believe that there was no 75 Summicron before the present Asph model. There was only the 75/f1.4 Summilux, which is now, sadly, out of production (prices are up, but you can probably find one used). I'm sure someone will jump in to correct me if I am wrong about this.
The current 75 Summicrons are, well, in the hands of their users. Used=being used (present tense). I got one about six months ago from a dealer in Hong Kong -- he said that production was backed up and that these lenses were trickling out of Leica. There were none either used or new in the US that I could find. Apparently Leica's cost cutting went VERY deep in terms of production staff, and now they can't meet demand quickly. And yes, it's a great lens -- simply great.
I have read very nice things about the performance of the C/V 75/3.5, but I assume that you would have purchased one of those if you didn't really want the Summicron. The current Summicron and the out-of-production Summilux are different beasts in terms of the look they give. The Summilux is an older (although very pleasing) design; the Summicron has more of the flavor of the current 50/1.4 Asph.