CNET has published a short list of items NOT to buy refurbished: http://reviews.cnet.com/8301-33153_7-57509549-10391733/three-tech-items-you-should-never-buy-refurbished/
Hard drives are listed as #1, and I would tend to agree. Data integrity is never something to gamble on.
Depending on the warranty offered with the refurb product, often the cost savings makes it worth the risk of “inheriting someone else’s problems”. I recently purchased a Dell XPS 15z laptop from Dell’s outlet (http://www.dell.com/outlet) at significant savings off retail price. The unit was sold as “scratch & dent” return. So far, I haven’t found any such cosmetic blemishes. I did have an issue with the display out of the box, but it was fully covered under warranty, and resolved in a reasonable amount of time. Other than that, it’s been a nice purchase and I’ve had no other issues (knock wood).