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Having six or eight processor cores on the desktop is a lovely thing to have, especially if you regularly use software that can take advantage of it. Re-encoding a massive movie file to a file size more suited to tablet or phone is far quicker, and it makes the prospect of encoding a huge movie collection a lot less daunting.
DDR4 memory is great as well. There's a big performance improvement over DDR3, and this translates to better performance all round. DDR4 will likely be a cornerstone of Intel's Broadwell platform, and future processor generations, so this aspect of X99 is a component that you will probably be able to reuse in a future upgrade.
The X99 chipset itself has some nice goodies and it's unlikely you'll run into a lack of SATA or USB 3 ports with it. This will be great news for people who like to build PCs with stupid amounts of hard disks. Then there's the nice SATA Express connector for faster SSD performance, which will become a growing market next year.
DDR4 memory currently commands a significant premium over DDR3. It makes the prospect of a Haswell-E system a lot more expensive. The great value of the Core i7 5820K processor is somewhat negated by this.
The single-core performance of all three processors isn't necessarily higher than the more affordable Devils Canyon processors. In many applications, that will have a bigger effect than the six or eight cores you get with Haswell-E. So it might make sense to save your money and go with an older chip.
And finally, while it's technically reasonable that Intel requires a new motherboard socket for its new processors, it's still slightly irritating that you can't just drop a Haswell-E processor into an older motherboard.
Minor quibbles aside, Haswell-E is a brilliantly fast computing platform. Having eight processor cores unlocks performance for consumers that was previously only found in workstations. You pay for the privilege, or course, but an eight-core Haswell-E system is still significantly more affordable than a Xeon-based workstation. Memory performance is great too.
DDR4 is the future, so investing in this memory now means you'll most definitely be able to use it for years to come. But as a closing thought, think carefully before jumping in right away. If you don't use software that will see any advantage from more than four cores, the only benefit will be bragging rights.