It depends upon where you buy them from - and how old they are. Systems that are 1-3 years old, and have some kind of warranty/guarantee from the seller, could be a good deal. Refurbished parts are usually factory certified again.
I purchased two Dell 19" LCD in 2003. One lasted until 2011, the other is still in use by my wife - it was priced at $199 - comparable monitors were going for $300-$400 at the time. I keep telling her she needs to update it....but she still likes it.
I have worked in retail electronics sales for years (20 years ago now), and do you know what the number 1 real reason for returns of electronics are? It isn't problems with the equipment - it is the "partner" of the person who bought it who finds out how much was spent on the item.
Legally, the item can't be sold as new. You have to test it, mark it down, and sell it. If it is returned to the manufacturer for a defect, the manufacturer must repair the item to be in working order.
The most common issue with returns/refurbished units are scratches or blemishes on the case and missing parts (i.e. a remote for a TV, cables, etc). You can open the box at the store and inspect the parts to make sure they are all there. Items with "major blemishes/dents" are not allowed to be resold without clearly disclosing the damage to the unit.