Originally Posted by Winter
I'm curious what constitutes a professional machine, a workstation. How long does a good professional machine last with the pace of technology.
Years ago, the distinction between professional-class and consumer-class was easier because there were bottlenecks everywhere. A workstation would allow faster hard drives, more RAM, better displays, faster CPUs and GPUs to the extent that you could not do certain tasks on consumer-class hardware. That distinction largely doesn't exist any more.
The only difference now between an iMac and a Mac Pro is that when it comes to raw computation steps like image rendering, the Mac Pro will finish the tasks in less time and the price seems to go up linearly (a $6000 12-core Pro will render about 3 times faster than a $2000 iMac - 3x the speed, 3x the price). These tasks can be offloaded onto dedicated machines though, in some cases, cloud services.
When it comes to longevity, Moore's law is adhered to for the most part but you generally see a doubling of speed every 2 years. It can come in uneven amounts, such as 25% one year and 60% in another to give 1.25 x 1.6 = 2x speed up over 2 years. It gets a bit screwed up when the GPU comes into play as Intel can allocate some of the 'silicon budget' to the GPU instead of the CPU and technically still adhere to Moor's Law but you don't necessarily see the benefit. At least with OpenCL compute, there's a better chance you will though.
Due to this doubling every 2 years, you can see that the highest-end iMac will outperform the highest-end Mac Pro within 4 years so that's the general shelf-life of the Pro in terms of superiority to consumer-class hardware. It's slightly less than that in fact as the 2012 Mini Server will outperform the 2009 8-core Mac Pro.
To justify buying a Mac Pro, you should expect it to out-last consumer hardware by 2 years so I'd say probably the fastest 6-core or an 8-core are the minimum requirements but it depends on what you use it for.
A workstation in 2012 is only beneficial for high-end rendering, compositing and possibly audio processing but these tasks were being done with 2009 Mac Pros and the processing requirements don't shift that much in 3 years. It comes down to a matter of pride in a lot of scenarios. A lot of users like to think of themselves as high-end users. Video editors for example could use a Macbook Air hooked up to a Thunderbolt RAID and 27" Cinema display to do their job but they'd get 'penis envy' seeing people in their field using E5 Xeon PCs, sign up to AppleInsider and declare that unless Apple sorts this out, they're going to switch over.
Computers should be chosen based on capability to perform a job, not based on preconceptions of what form factor is required to do them. The Mini Servers and iMacs are perfectly capable of doing almost any high-end task. To justify a Mac Pro purchase, you have to go up to about $3500-4000 + display.