I just got my first set of Denise Interchangeables (the store was out of Knit Picks, and I needed some instant gratification), and I am pleased with them. I also have plenty of addi's, and recently bought a pair of Size 13 Silvalume's. Mine do say made in SC (and I like that the Denise's are also made in USA), and they may have made an improvement on the silvalume production as the join is perfectly smooth on the pair that I have and does not have a crink where the needle and cord come together.
I love my addi's, that's for sure, and look forward to getting knit picks some day. In the meantime, I can only say positive things about my new pair of size 13 silvalume's that I just bought. I'm on my second project with them. I do agree with Ingrid; they are slightly less 'slick' than the addi's... I can get over it, but I noticed the same thing as well. And I also like the fact that addi's have a guarantee, as one time, I actually did have a cord separate from a needle, took it back to the store, and it was replaced immediately.