It depends
For a workstation, compared to say the Dell T7610 or a HP Z810, the nMP offers good value, but at the cost of expandability. Look at Anandtech's review and there are limitations to the current Intel workstation chipset. With the nMP most of the bandwidth goes to the graphics cards, and rightly so for a workstation aimed at the professional video and graphics market. If you need more CPU or want 10GigE, or want to connect to FC storage the nMP might not serve your needs so well.
As an general purpose desktop I feel the nMP represents poor value for money. Most of use don't need workstation class components, but if you don't want an iMac you have two choices the mini and the nMP. There's a hell of a price difference between them that can't be ignored. I've just bought a Core i7 Mini with a Pegasus 2 array. The Mini will be fine for most of my tasks, but not for gaming. I'll be keeping my PC for games until a point where the mini's integrated graphics are good enough for gaming, or there is a a new product based on the nMP or the nMP has more configuration options. At that point I'll ditch the PC and mini and replace them both with the one machine. The storage and my display will stay. Why bother with all this? Because I don't feel the nMP represents good value for my needs, and it might be better to just replace mac mini's every 2 years or so.