The absolutely very last thing on earth I want to do when I come home from work and want to use my computer is mess about with it, worry about software updates breaking it, have to assemble it in the first place.
The £1640 I spent on my 27" iMac with 680MX (edu pricing) is well worth it for me. I will run OS X, it will run modern games (in OS X and via bootcamp) and will be with me for at least 5 years. It is replacing a 2006 iMac that I spent £1100 on all those years ago. For the price per year of the "genuine" machine, for me there is simply no question that it is totally, totally worth it.
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Very few people need hyperthreading.
The only real benefit of the i7 over the i5 is that slightly higher base clock speed.
For the price delta for the i5>i7 upgrade ($200), it's simply not worth it. By contrast, the delta between the 675MX and 680MX GPU at $150 is a marked increase in performance, as is the performance increase for the money to get the fusion drive.
Very few people really need the i7, and the cost saving for leaving it out still gives you almost the same performance unless you are doing very specific things that take heavy advantage of HT or constant high workloads on all 4 cores.