My personal preferences are Hiya Hiya stainless and Knitters Pride Dreamz, both of which use plastic cables of some description (though I couldn't tell you what the construction is). Hiya Hiya's interchangeable sets come with plastic shrouded braided steel cables, which I haven't used but wouldn't imagine would be terribly friendly. They wouldn't have any "memory" to speak of, but I should imagine they'd be stiffer than I'd like. The Dreamz interchangeables use the same cables as the single-length models. My girlfriend has both sets of those and LOVES them. From what I can see, the Knitters Pride sets are pretty much the same as the Knitpicks, except maybe in the type of wood used for the tips.
No doubt there have been or will be lovers of Addi chiming in here, and they make a good one too, I've just never personally been impressed enough to pay the premium for them over the others.