Have one on order (via overseas) - 6 core, 64Gb RAM, 1TB SSD, Dual AMD FirePro D500 GPU's.
Pound for pound vs a PC build, I've figured the price is actually fairly comparable, albeit mainly because AMD are literally giving away their graphics cards, which heavily offsets Apple's steep markups elsewhere. Not that I'd use a a Windows PC anyway, I prefer to have a life
Will be using it for high resolution photo processing and some video. Given that Photoshop, Lightroom etc dont utilise more than 6 cores (yet), I didn't see it feasible to go for an 8 or 12 core, knowing that the processors are indeed interchangeable (which silenced a lot of naysayers assuming Apple was to be welding them to the motherboard). I also decided to stick to the standard GPU config, because the ability to run six 4k monitors isn't exactly a priority for me!
In hindsight, I probably would have spec'ed the order even lower, and upgraded the SSD and RAM via a 3rd party source, but I impulsively pressed the Order Button. There is however the risk of warranty complications with the likes of OWC. It once took 4 months for them to replace a faulty RAM card I'd sent back to them. Good thing is with the new imported MP is that I can have it repaired locally under warranty, whereas this previously hasn't been the case with non-portable Apple computers.
As for the potential of a messy desktop with peripherals. Yep, it has me pondering, but I think with careful planning and organising this shouldn't be too much of an issue.
True, this machine is complete overkill for most people and no surprise they see the price as eye watering. I on the other hand have been patiently waiting for a much needed upgrade to my (now kaput) 2008 MP, so to me it's feasible.