In my opinion, if you can get a refurbished one, make sure that it's factory refurbished. Going to a local dealer and getting components that he's "cleaned" or a PC that is freshly formatted is not the answer, you're always better off with a new PC or component in that case. However, I've had great luck if I'm ordering from a manufacturer and they offer a great deal on factory refurbished products.
i once purchased a used laptop only to discover that it had a connection problem and the batter didn't charge, it only ran on AC power. This was super annoying for me. I had to super glue the cord so it wouldn't accidentally fall out because it was a fairly loose fit. It worked fine other than that though. I would not buy another used computer for that reason though, if I had a problem first time around, I'm likely to have a problem the second time around. I have seen a number of computer advice sites that discourage buying used computers, but then many people have been happy with them, so it's up to you.
I only buy new for electronics- I know plenty of people have decent experiences with buying used or refurbished but I would be really hesitant to. A lot of the time you can find decent deals for new computers if you're willing to shop around a little.
Depends on what you mean by used. If you mean that it was sent back because of packaging issues and it works fine and you get warranty for it, then why not. But if it was used for more than 3 months I wouldn't buy it, even if warranty is given, as it is more exposed for having problems with in the future.
To the above poster, there is warranty on refurbished computers depending on the discretion of the manufacturer (but in most cases, there is warranty). In my experiences, refurbished computers tend to either have no problems or very minimal problems. However, take this with a grain of salt as all manufacturer's refurbish process is not the same... Some check more than others and are more careful in what used products they release back to the public.