What you have to watch out for are things like batteries on their last legs and can you even get a new one if needed?, CD/DVD running OK?, and so on. It all comes down to the price and what you want to do with the laptop. Basically all i do with mine anymore is surf the web. It is an older model I got on clearance a few years ago. I loaded up Linux on it and it still runs like a champ.
Once again, depending on what you want to accomplish, you may be better off going for one of the low end $299 deals you see at various stores. They are surely to be more powerful if nothing else.
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).