The most common argument I hear against those who oppose embryonic stem cell research is one that is, to me, completely vapid. It basically goes, "we could save a lot of people with this research, so therefore we should do it."
This argument makes one of two assumptions, as best I can tell. It could be assuming that the embryonic life (and it is a unique human life, biologically speaking; this is a scientific fact not seriously questioned) is not a life that deserves any protection at all. This is, of course, classic question-begging: this is the very crux of the issue, and so assuming it is nonsensical.
The other possible assumption -- sometimes stated explicitly -- is that even if these are lives, it is acceptable to kill them, because other lives are worth more. Humanity has gone down this path before, and I refuse to. It's anathema. And its atrocity is compounded by the fact that it's the government choosing which lives are more valuable.
So please, save your breath. Don't say "we should do this because good may result." (And worse, don't tell me that good will result, because no one can know that.) If you want to convince me, you cannot possibly do so by telling me the potential benefits; you can only do so by convincing me that no human life is being intentionally killed for the sake of research. I don't care if you convince me that we will cure AIDS and cancer tomorrow by killing off a few dozen embryos today, I will oppose it, if I believe, as I do now (because how could I not?), that those are human lives. Period, end of story.