I always figure " go for it"!!  If it really is a dud and it's a respectable company then they'll help you out on a new one.  If they won't then there's always the mfg where you could RMA it.  I got a refurb HD at one time and after a few months of good service it gave up the ghost...probably not even related to the fact it was a "refurb" since these things do break.  I called maxtor, got an RMA and in 3 days had my new drive.
Refurbs IMHO are a good way to save a few bucks.  What usually happens at the retailer is that they get this item returned for whatever reason; usually not because it's defective but because somebody figured out they didn't want it or spent too much.  Once returned they need to recoup some of thier losses so they sell it to me....as a "refurb".  I never really trust that anybody really "refurbed" it and checked to see that is was good, but they just resell it and take thier chances about my furture problems...