Hmm. I've taken two SAT subjects tests and got high scores on both (although I want to take a couple more, Physics and Chemistry. I've taken Math II and Biology M. I can raise my Science SAT II by a little by taking those two.)
As for schools I'm looking at, there is MIT, Caltech, Stanford, and other big math/science schools. If taking the test is unnecessary, then I don't think paying for it and taking it would be worth it. I might take it though, because it is "nice to have." I'm still debating as to whether I should, which is the reason for the thread in the first place.