I support an op-out system, but with no pressure to donate while you're alive, if that would potentially be part of it. I believe it's very different to not necessarily want to donate an organ while you're alive, and feel that way myself, although not at all when it comes to blood. I would donate an organ to a dear friend or family member, but the idea of giving up something my body can still use and can't replace while I'm alive is serious enough to me that I wouldn't easily do it for a stranger.
When you're dead, however, I think it's selfish and silly to care what happens to your organs, outside spiritual reasons (pretty much the only reason I would have the option to opt out of it re: after death). You aren't using them, you're letting them rot in the ground or be burned to ashes while someone else could be given another chance at life with them. This rings particularly true for me in the case of people who need organ transplants for reasons that were always outside of their own control.
Whether opting out should make you ineligible to receive transplants... Well, I can see both sides when it comes to that. It seems callous, but at the same time, why should you get help you yourself refused to give?