I purchased a Canon 17-40 as a refurb and it has been nothing but great. While the cost difference was quite a bit more than $50 from the new one, I feel I made the right decision.
I would be hesitant to purchase a body as a refurb, but that is just me. On the lens, lots of times, you can talk the dealer down a bit on refurbs just like on used.
One thing to think about is the warranty. For $50 it might be worth it to get the warranty as a comfort barrier. My CPS membership helps me out on stuff like this, but it is always nice to have a warranty to back you up.