A pro with serious workstation needs reviews Apple’s 2013 Mac Pro_Ars

The Good

Impossibly quiet operation, even under load.
Power consumption at both idle and full load is amazingly low.
Space-saving design is a marvel of engineering.
Dual D700 OpenCL scores are incredible.
A good upgrade over previous Mac Pros for poorly multithreaded programs.
PCIe SSD is exceptionally fast.
Thermal temperatures are safely in spec for extended workstation-style usage and GPU operations.
Good pricing for workstation-class GPU options.
Affordable relative to similar configs offered by competing workstation vendors.
CrossFire enabled in Boot Camp so you can get very good gaming speeds.
The Bad

Drivers are to blame for some very low OpenGL results for the FirePro D700 in OS X.
Reliance on external devices for PCIe expansion increases cost and has some short-term compatibility implications for some.
Lack of Nvidia GPU option and CUDA problematic due to Apple’s lackluster OpenCL developer support.
If mid-life GPU upgrade kits aren’t offered for these machines, they will age badly for 3D and OpenCL work.
Despite the FirePro logo in the Windows AMD Catalyst utility, GPUs appear as Radeons in Boot Camp, so it’s not recommended for Windows pro apps unless a FirePro driver becomes available for these GPUs.
The Ugly

Lack of dual-socket CPU options means that the 8-core Xeon E5 v2 gets the same multithreaded CPU performance as the mid-priced dual-CPU Mac Pro from 2010. It’s bested badly by more recent dual CPU workstations, even for the 12-core.
If you are using programs that are poorly multithreaded throughout, then an iMac is frequently a better option due to faster clock speeds.
— http://arstechnica.com/apple/2014/01/two-steps-forward-a-review-of-the-2013-mac-pro/