Ive done a lot of real-world benchmarking with current gen hardware, as I make hardware purchases for high-end media production and have recently bought a lot of systems. My results:
The Mac Pro is a good buy for Final Cut Pro Pro-Res editing (software is tuned to those OpenCL cards). It can also be valuable for aesthetic purposes if you want to look cutting-edge to clients. For everything else, its not a very good value.
For our normal sub-$4,000 stations, the 27 iMac far outperforms the Mac Pro, especially in single-core but even in multi-core (Mac Pro doesnt give you many cores until the price tag jumps up). The iMac is great for video editing, sounds design, and iOS development.
For our expensive stations where we need tons of horsepower (3D, VR, IMAX, Color Correction), HP Z440/Z640/Z840 workstations offer far more performance per dollar. You have more CPU choices (as well as v3 Xeons), and you can use monster Quadro cards or even GeForce if youre willing to go outside qualification (Im currently buying GeForce GTX 980s for their virtual reality latency-reduction features).
Since almost every pro app we use is OS agnostic (Avid/Pro Tools/Unity/Adobe/Maya), I go with HP to give users the most power. Given what Apple has done to Shake, Color, and Aperture, Im not sure why professionals are still loyal to OSX. Im not an Apple hater, as I buy their hardware often. Just not for high-end use. I find the Mac Pro to be a strange beast, so I highly discourage it outright in our environment. I'm also sad there's no longer a good hardware choice for a Mac server.
I welcome feedback, as I try hard to consider every user when I make recommendations.