The thing is fast. The SSD tests at 800MB/s write and 932MB/s read. The external drive is 318MB/s write and 370MB/s read, which is as fast as a cheap SSD. 60GB/s memory speed (1.867GHz x 8 bytes wide x 4 way interleave). The Intel E5 is fast in a different way than the i7. Even though it is only running at 3.7GHz, it doesn't slow down when you add more applications.
Money well spent. Lots of money. $2.8k for the Mac, $680 for memory, and $400 for the disk drives. We paid $3500 for the top of the line iMac five years or so back, so this wasn't extraordinarily expensive. The money spent on the 4790K isn't a loss, either. That box is now running Windows 8.1, and doing (of all things) Mac program development with Embarcadero's XE7 (C++ Builder). I knew Windows 8 had to be good for something.
I've had a few of months with the Mac Pro and I have decided that it is even better than I thought. I have it set to spin down the external drives when they aren't being used, and although it sometimes adds a slight delay when I save a file, it isn't at all annoying. When it wakes from sleep, there is a delay in network access while it brings the network back up. Overall, the machine acts like a big rock rolling down a steep hill. Nothing can slow it down.
Although it can handle three 4k or six Thunderbolt displays on its two graphics cards, it can't handle 3 HDMI monitors. That is apparently because only one graphics card is used for the main user interface, and it has just two "passive" (HDMI/DVI) outputs. If I add a $30 active HDMI/DVI adapter I can run an additional monitor. Or if I add a Thunderbolt monitor. I would love three Thunderbolt monitors, but even one is more than I can justify. And I share monitors between the Mac, Linux, and Windows machines, so I need monitors that are compatible with all of them. The Windows box has a display port connector, but I haven't seen a display port switch.
One more update. It's been a few years. I upgraded the CPU, so instead of a 4-core 3.7GHz E5-1620 v2 it has a 6-core 3.7GHz E5-1660 v2 CPU. It was nerve-wracking taking the little screws out and completely dismantling the computer to get at the CPU. I was hoping all the time that I didn't lose a screw. For the first time in my life I neither came up short of screws nor had a surplus. It runs just as fast in single-threaded tests, and 50% faster in multi-threaded tests. It was worth the trouble.