"Blonde On Blonde" is one of Dylan's three best albums, along with "Highway 61 Revisited" and "Bringing It All Back Home." This is widely agreed, and I won't write yet another review of this classic -- suffice it to say that if you have not yet heard it, you have quite an experience to look forward to. (See my "Memphis Blues Again" list of Dylan's 10 best.)
My review is for those considering an upgrade. I had it on wax for years (since 1974), and then the original CD. I was wary about the long-awaited remasters, given limited time and money and the amount of music yet unheard they are competing with -- I finally decided that if there was one Dylan album I would most like to hear with state-of-the-art sound, it was "Blonde On Blonde." Hoping to be astounded by the difference when listening to the original CD and the new remastered one back-to-back, I was disappointed. Yes, there are places where there is more detail, but on balance, my conclusion is that the slight improvement does not justify the expense. So my recommendation is, unless you have expensive enough equipment to maximize the SACD format, the old CD sounds just fine.