I find them a bit meh, tbh. The ichthus fish was originally a secret symbol type thing for persecuted Christians to identify themselves. It's really nothing at all to do with creationism, so I find the darwin fish a bit odd, really, like they're trying to be offensive, because Christian doesn't equate with creationist.
I believe in special creation. Haven't a clue how it was achieved, but you know, all powerful God, finite me, I don't think it matters in the slightest.
So, as a Christian, I believe in the reasonable law of the land, so even if your were to put a sticker of a cross on your car that says 'If you're a Christian, you're an idiot' I wouldn't trash it.
Might not feel like smiling at you as you drove past, though (although I probably still would, because I'm a bit ditzy like that).