I signed up for my organs to be donated before the opt out system was implemented, and I think this will help ensure that waiting times for organ transplants will decrease. I have warmly welcomed this change ever since it was announced.
That said, I think we should respect people's wishes to opt out if that is what they choose. The wishes of the dead in regards to what happens to their belongings is taken seriously in all other contexts, even if it may seem ridiculous to us. For example, if a man with terminal cancer felt that his Bentley ought to be buried alongside him upon his death, as stupid and wasteful as it would be, it is still his decision to make.