I've owned one Dell refurb, and referred them to several others as well (from their "outlet" website) over the past 10 years or so, all to good effect. Dells may be a bit different though, since many of the ones they sell on the outlet site are ones that were returned having never been used (corporate customers over-buy, then return what they don't deploy, etc...).
I haven't used any other brand's refurbs, but would be perfectly willing to pickup an ASUS if I needed a new one.
The only negative thing I can say about refurbs is watch out for the Macbooks that are available for $400-$500, since they are usually in very bad shape (read around the web, you'll see what I mean).
Good luck.