OMG. All the incessant bitching about price. This thing is a friggin steal at $6k!
March your little whiny butts on over to hp.com, and custom configure yourself an HP Z4 (nice machines btw) with identical specs to Apple's entry level config (or as close an equivalent as possible) and then ask yourself if you have any clue how much workstations cost!
I just did that very thing... Matched up an 8-core Xeon CPU, 32GB RAM, GPU, PCIe based SSD, etc. Was honest and remove things like standard 1TB HDD, CD drive, keyboard, etc. to match the config of new Mac Pro. The result? HP Z4 = $7221.00.
Now shut up and get back to work.