What people also forget is that just buying a Mac Pro isn't enough, unless you sit on a big infrastructure with NAS and all. You may need to buy lots of addons, including thunderbolt boxes for adding storage and PCI cards. Also not optimal when you want NVIDIA GPUs, or can actually go with just one GPU. So, the final bill (and the footprint) adds up, and you'll soon look at 6k+. Plus, not much to upgrade - essentially a throw away device.
On the other hand, a OCed Sandy or Ivy Bridge E can beat even a 8 core XEON, you can put more RAM into a decent X79 or C60x board, you can configure the machine to your needs. You can upgrade and even fix the machine by yourself and reuse components, even beat the GPU power of each and every Mac Pro by using quad sli setups etc
So, with a decent board (R4E or P9X79-E WS, which also takes XEON and ECC RAM), a 4930K or 3930K or even one of the same CPUs the new MP uses, 32 Gigs of RAM @ same speeds like the mac pro and way more SSD and HDD storage, plus one or more decent GPUs of your choice(!!!) you will tick at around 4-5k. This machine should easily beat an entry level nMP and be roughly on par with a midlevel nMP. And yes, that will include a case (of your choice) and a PSU (of your choice). What you will loose is ECC RAM, Thunderbolt and it will cost you a bit less money (but not that much), but _way_ more time in many cases to get everything right. Be prepared for that! Just my own experience and 2c
PS: and yes, Rampage is right about constraints: also think that the NMP will have thermal and power restraints just by the way it is designed. Quite curious what happens if you actually put this thing under full load for a longer time. Would be amazed if it does not throttle back in any way.