Apple products are really the only refurbished products I buy (aside from cars). Almost all of the current models that are refurbed are CTO (custom ordered) Macs that were returned. So someone buys a Macbook Pro with more RAM and changes his/her mind.
My wife and I have ordered a few machines this way and have been surprised to see that the specs are better (more RAM more HDD space) but that was years ago, mileage will vary.
Others are returns without boxes, and other items, returns that were said to have a flaw but don't, or returns that have minor flaws (key pops off, dead pixel) and are repaired.
None of the systems have flaws or huge manufacturing defects. Those are parted out for Genius Bars.
Older systems are machines that were new that the retail stores send back to Apple, like any and all 15" MBPs that were in stock before the 16" hit the market.