Guys,
i work with huge files that benefit a lot from SSDs (streaming big audio samples), and looked hard at the options.
The Apple one is just not wise, and should not be pursued no matter what i believe.
If you get a "basic" 1TB fusion drive you'll have the OS and most of applications on the SSD, leaving some space left for storage of other files in case you want to use it. You also can't, limiting the space you use in your Mac so to be sure everything inside is "SSD only".
With the remaining money saved from the Apple option, you can purchase, for example, a 1TB SSD Thunderbolt from, i.e., Lacie (33% more space than Apple option, and looking at MBPro Retina benchmarks, probably faster).
It's still obviously very expensive but:
1- You get more SSD GBs (33%+)
2- You get faster performance driven by both faster SSD and the fact that you have two SSDs you are using instead of one (the one in your iMac and the external one), so there's no competition between the two.
3- You get storage space on a regular HDD already built-in in your Mac in case you need it.
Net, I can't see a way to justify the expense whatsoever, just as much i can't justify buying into Apple RAM upgrades (the single truly rip-off of the company that i believe should be borderline to illegal, we're talking about a 300% mark-up on a commodity that is available everywhere, if only all consumers knew).
