I'm not a fan of the Surface Pro 3 as a replacement for my Surface Pro 2 because the Surface Pro 3 is too big for my taste.
Having said that, I have to investigate some more but the SP3 might be faster than my 2010 iMac with an i5 quad core, and if that's the case then I might buy it to replace my MacBook, iMac and Surface Pro 2 (although grudgingly for the latter).
I'm not sure how this will work out. Any serious work or gaming on the Surface results in immense heat, my SP2 has gotten up to 60C which suggests to me that a SP3 as a desktop replacement might end very badly under heavily daily use. The SP3 is supposed to run cooler than the SP2 (I think), but I don't know if this will make enough of a difference to matter.
Edit: apparently my iMac from 2010 outshines the SP3 so I might not buy it now, not unless OS X runs well on it anyway:
Intel Core i5-760 vs i7-4650U
tldr;
under heavy use the Surface heats up quite a lot, if you're uncomfortable with that don't use it as a desktop replacement.