Really, ideally, every drive on every computer should be an SSD. It's faster, more durable, and more reliable. It's ideal. But sadly, it's not cost-effective yet. That being said, the Fusion drive set-up, for someone buying a new Mac today (or even reformatting and re-setting up an older Mac with both an internal HDD and and internal SSD), the Fusion drive is a great way of having the best of both. Yes an SSD is marginally faster (but really, only by a hair) and using one drive instead of two is way more reliable (but that doesn't matter if you have at least one Time Machine back-up). I'd say, for now, for this generation of iMacs and Retina MacBook Pros, a 768GB SSD isn't worth it. Even if you have the money, it's better spent elsewhere. And if you have to spend it on Apple stuff...put that towards your eventual next Mac.
I find that Apple has me somewhat blocked in a corner with their current drive choices. I need about 400GB for my normal OS X environment, plus another 400GB for my photo library ... plus I want a BootCamp Windows environment for a bit of game playing with the top graphics card in the 27" iMac. And, I want it all
internal to the all-in-
one iMac, no boxes, cables, power supplies, etc. all over my desk like my Mac Pro has.
I want my OS X running at SSD speeds ... and I want my Windows running at SSD speeds ... plus I want to be able to sort my photo library at SSD speeds.
I can configure the 768GB SSD into a 200GB Windows boot disk and allocate the rest to OS X with a comfortable growth space. But for now, my photo library will have to be external on a 500GB SSD with USB-3 or Thunderbolt. However, if/when I care to open up the case (warranty?), I can put that 500GB SSD inside mounted in the location where the hard disk would normally be.
So ... buying the 768GB SSD does make sense for this type of configuration, even though it is slightly expensive (find another
single-drive SSD of this size that is cheaper today?). There is no other solution available today which will allow an all-SSD system of this size to be contained within the iMac case.
I have run Fusion systems ... I am running 3 DIY Fusions at this moment in a Mac Mini, MBPro, and Mac Pro. The Mac Pro also has the all SSD environment I have been running for the past year. I can tell the difference, and the all-SSD is worth it to me for the additional cost, which,
amortized over the next 2 or 3 years is insignificant. Many probably spend more than that on coffee each day.