If it doesn't take long to opt out, then it doesn't take long to opt in either.
it's quite a weak argument though, tbh. The reason opt-out is being seen as a better of the two alternatives is that opt-in wasn't bringing anywhere near enough organs. As with premature babies, the further our medical knowledge and abilities advance, the more people who would previously have died are now living longer lives. The quality of those lives is rarely debated on a superficial level, but it is still part of the discussion.
Liver and kidney transplants are pretty routine now, heart and heart & lung transplants are on the way to pretty much routine. If we want to accept that we have medical capabilities to do that, we have to find a way to get those organs and asking people to donate by means of an opt-out is probably the least-worst way of getting more organs knowing that opt-in wasn't cutting the mustard..
What we don't really have is a good end of life discussion going on, in families and within wider society. Some people do, and if you've ever had a family member go to a hospice or end of life care, it can come as a massive shock to meet people who talk about death as part of their daily lives.
As far as I'm concerned I'm fine with opt-out (I was also pretty fine with opt-in) but i want it to be impossible for my family (or anyone else) to override my wishes in this respect (which ever way i choose).
I also want there to be a (insofar as possible) watertight end of life protocol for removing life support, with again my wishes paramount over my family if i have made them known. We should also all make wills and living wills as part of our regular routines, reviewed regularly and particularly upon life changing events (marriage, birth of children, death of spouse etc etc).
Given advances in medical matters i also want guarantees (cast iron as possible) that wealth is never going to be a determining factor in selection of donated organ recipients. In fact this, for me, is one of the most important aspects of it. Emotionally, i tend to the "if you're not a donor i don't want you receiving organs" but pragmatically i know that this isn't the best way to select recipients and i don't want that to play a part.
Sometimes i think it must be so much easier if your faith or beliefs rule out things like transplants and blood transfusions. You don't have to make any decisions because they are made for you. And then i think, we have free will and good brains, we must be able to come up with a good system.