So long as you know the specs you are buying (i.e it will likely be a slightly out of date or discontinued model) then no problem. I have had very good experiences with refurbs, my newest macbook is a refurb and I've had it less than a week, very happy with it and with the service from Apple. If there is something you are after, keep an eye on the Apple refurb store for a few days or a week as stock comes and goes.
Make sure you're getting the latest model; the discount for older models isn't enough in my view. Otherwise, excellent. You don't get the original packaging, but everything else is there and you're still covered by Apple's excellent customer service.