While I can afford the 768, The iMac I have on order has the 3TB Fusion and this is my thoughts:
1. If Apple had offered a 256, 512, or 768 Flash drive + 3TB 7200 HDD I would have gone that way. The way I see is that in 3 years, I am going to rip it open and upgrade the Flash SATA port to whatever the max in 3 years is, and will replace the 3TB HDD with a large SSD drive or maybe a monster HDD..cross that bridge when I get there. So the 3TB fusion gives me 90% of what I was expecting to get anyway.
2. The price of the 768 Flash upgrade is out of control in comparison to what else I can buy with the 900$ difference. While I would LOVE to have 4TB of flash in my desktop, I think we are still 2 generations of computers away from that time. In addition if I go 768, I will have an unused sata port inside the iMac for 3 years and it's a desktop not a notebook, so I will expect to have at least 2 drives inside it maximizing my storage.
3. I can drop the 900$ into a Drobo 5D, add flash cache to it and start to fill it up with whatever drives I have laying around till it's 100% SSD. In less than a year, I can have 5 SSDs inside it, with loads of fast redundant thunderbolt storage space
4. I'm disappointed that my Fusion drive coming in my iMac does not have a 256GB Flash drive and 3TB HDD
5. Apple clearly wants us to buy the fusion drive...just take one look at the options and prices. With my system on the SSD and cache'ing via Fusion, I do believe it's the best option weighing cost and size
6 My 2013 MacBook Pro with Retina will have a 768GB flash storage or whatever the max option is at that time...
7. My music collection is 400GB, so it's not like I can only have 768GB of internal desktop storage
So these are my ideas, thoughts, and reasons... have a great day everyone