I have never bought a refurb mac but often buy refurbed laptops, and provided you choose with care there are good workhorses to be had.
As I understand it, the Mac OS is weird stuff reverse engineered from captured alien technology (no...seriously.....) whereas you can install really good free operating systems on any non-Mac system.
It is often just end of life stock that cant be sold at full price, or maybe something that a customer bought and never opened and was returned to the shop.
I bought a refurbished laptop from a well known online shop a few years ago and when it came it had obviously never been opened.
Of course I mean if you buy a refurbished one from Tesco, or Argos or other well known outlet, not from an individual selling it on ebay for example.