I ordered the 6-core Base model, only upgraded the drive to 512GB. Will upgrade the RAM later when 3rd party RAM prices come down and when doing so will help me feel like I am getting "oomph" for my system and stretching its life. Next in line would be swapping out the internal drive. Maybe by then, I will be able to get something bigger than a 1TB for a decent price. If not, I can live with 512GB for a long time, and in 2-4 years even a really fast 1TB option will likely cost just a couple hundred or less.
I had almost purchased a loaded 27-inch iMac last spring. Then I heard new Mac Pro coming. Decided to wait. Glad I did. Like many others in this thread, I don't need the Pro. My lack of need, though, is outweighed by the fact that I never really wanted an iMac. It bothers me that I must have Apple's monitor. In a word, lame. Yes, I could get a Mini. I don't want one of those either--too many limitations. I shoot RAW pictures with a pixel count of 6016x4016 (Nikon D600). I'd like to get as close to actual size viewing as possible (so I can see the oil spots in full detail!). When 4K comes out, I will be even closer to that. If I buy a Mac Mini or an iMac, I'll be stuck with its current top resolution. That's why I need a new Mac. I bought my previous iMac back in 2008. I've gotten a lot of use out of it, but it won't even display full resolution on my recently purchased NEC PA271 (2560x1440). I'm not complaining. I knew it wouldn't but the monitor was deeply discounted right before its replacement came out and I knew I'd be buying a new Mac soon. Anyway, if I am going to pay $3,000 for an iMac, I might as well get something that will be far more likely to satisfy my needs in 3-5 years. I am pretty sure an iMac, even fully loaded won't. In three years, I may upgrade my camera to whatever comes after the Nikon D800. That may have a pixel count of 9,000x6,000 or more. I am fairly confident that my nMP will be able to handle whatever it is just fine. I am also fairly confident that a current iMac will not. In 2-3 years, I will want a high-end monitor to add to my current monitor (the current monitor will still be good for print proofing, color correction). Again, whatever resolution it has, the nMP will probably support it, and the iMac will not.
So, it isn't just about speed now, it is about not wanting to worry about getting another computer for several years, and given my needs, that is a priority. I can really see this computer meeting my (admittedly non-pro) needs for the next 5-7 years, or more.
After ordering my nMP, i realized another unintended benefit. I have a house, and I have a cabin in the north woods (where I am right now and will be, for the next two weeks). I do photography (Aperture) and music stuff (Logic, Native Instruments stuff, occasionally Reason) in both locations. My "powerhouse" computer right now is my 2008 24 inch iMac. It is at my house. I would never consider moving it. It is heavy, bulky, and difficult to move around (other than swivel and tilt). I bring my MacBook Air to the cabin, and occasionally use it for Logic, but it starts to choke if I throw too much at it. Also, most of my instrument samples are only on my iMac because the MBA's hard drive isn't big enough. I also don't download my photos to my MBA because of the limited size of the internal SSD (I bring my cards home to straight to my external HDs and back them up to my Time Capsule Drives). Now, at 11 pounds, and with a built in handle, I can easily see moving the nMP between house and cabin. I am sure padded cases will be available soon. I can just leave an extra power cord up here, have a TB2 External HD with all my samples and instruments on it, and throw it (put it gently) in the case with the Mac Pro, and I have a very powerful portable recording studio. I already have two full sized keyboards, one in each place. I also have a second monitor at the cabin for extra screen real estate when I do use Logic and Aperture here. This is going to make things so much better!