I've got a refurb NOOK Color and a refurb NOOK Simple Touch. Both work flawlessly. The only thing I've noticed is that the little 'n' button is recessed a bit more than on my new units. On the NST, it's fine, but on the NC, it was almost enough to make the button tricky to locate without looking.
Just give the replacement a quick visual look before accepting, and you should be fine.
Several members of my family have purchased refurbed NSTs and haven't had any problems in the months that they've had them.