I try to stay out of these anymore.... but I'll chime in once. The systems Signburst builds as well as true Professional workstations from Dell (their Precision line), HP, and IBM are in a totally different class than any "off the shelf" consumer systems. As Casey has mentions they are built with components that meet a far great stringent standard of quality and reliability. His example of hard drives is a good one - take 2 near identical Seagate drives and compare their consumer drives specifications, warranty, predicted MTBF (mean time before failure) and see what you find. The enterprise class drives are rated for often double the life with double the warranty. Manufacturers of chips and other components use a system called "binning" to determine capabilities and qualities of items. Higher binned processors for example might be capable of crazy overclocking but they are not specified as an enthusiast processor, they are configured internally and coded as a Xeon model processor instead. The parts that are still good workable parts (might not have quite the long term reliability or capabilities) and make up the majority of the parts made are still used but sold as the primary consumer line products while the premium top shelf parts are sold at a higher price point with better warranties and life ratings due to their premium qualities. These are the parts that are generally used in professional and enterprise class computers. Professional workstations are also very carefully engineered for optimal cooling, all the parts are very carefully and meticulously tested together to be 100% certain that there are no compatibility issues, etc. ECC memory - another plus. If there is some random software error or memory error - ECC memory corrects it and you never know any different. Standard ram, well your program probably just froze, maybe crashed, maybe Windows blue screened, etc....
Building your own system.... well that compatibility testing is you putting the parts together and hoping for the best when you hit that power button and when you start loading software onto the system. Did you really look at the specs of that cooling fan you picked out for the computer you are building to see what type of bearing it uses, its CFM rating, its noise rating, and what its amperage draw on the system board will be? Really? I thought so.... Don't get me wrong. I've built hundreds of systems over the years too. Most with no problems. But I'm not going to kid myself thinking that they have the proven research, time, effort, component selection, etc of a true professional workstation like what Signburst is going to build or like a Dell Precision does. The uptime on one of the Precisions here is over a year now, who care if it can reboot in less than 30 seconds if it has not rebooted in over a year? Granted that one is the dedicated RIP that is not used for web or anything else (its firewalled on a dedicated private subnet) so it has little need for Windows updates or anything like that. Even my main design station which I do keep reasonably updated will generally only get rebooted maybe once in a month when some big Windows updates require it.