This article, by an oncologist, lays out the situation quite well:
Patients trust their secrets to doctors, not the government or the tax office
I have a friend aged in her 30s who for the last eight years or so has had what she describes as "chronic cancer"; a slow moving cancer which has metastisized but doesn't look like finishing her off for quite a few years yet. Her doctors play whack-a-mole with each new complication as they arise and obviously proper communication between her many specialists is vital. For her, a "My Health Record" must be a no brainer; the advantages are enormous, as trumpeted by the systems supporters, in spite of the worrying privacy implications.
I, on the other hand, have been comprehensively screwed in the past by the medical fraternity and have excellent reasons to want to maintain an iron grip on who gets to see what from my medical records. I opted out within about 10 minutes of reading the first post of this thread. No matter what the supporters may think of the advantages, the downside of potential loss of privacy is simply far too damaging for me.
The article referenced above basically says the same thing; the concept is very attractive and will undoubtedly improve healthcare for many participants, yet the execution, the security, and the potential for future access by elements of government who have no business poking about in your medical records are serious problems.
The good bits are real, so are the bad bits.