I currently own the knitpicks options and the denises. I also owned the boye's and disliked them.
Knitpicks: I've done 2 projects with them and LOVE the flexible cables. I love the smooth metal needles and I really "fly" when I knit. I WISH there were smaller cables and a few smaller sizes, for knitting socks on 2 circulars for example. I wish the needles were labeled in a way b/c I have a hard time telling the sizes apart.
Denises: I have the colored set that was done by Amy. It's very easy to tell them a part and the joins are easy and I haven't had any problems with them coming apart when I knit. The cables are a bit thicker than the knitpicks options but they are nice as well.
Boyes: I DID like the fact they were each a different color so it was very easy to tell what sizes I had. I HATED the cables. They were SO thick and not flexible. They actually caused my wrists to hurt when knitting in the round ( I did about 5 booga bags with them).
So there's my beef. I Hope this helps others when they are shopping for interchangables