I bought the 21 and the 27 inch refurbished and I cannot tell the difference between them and new machines, and I have inspected both of them thoroughly. Not a single scratch, dent, etc.
I go through a lot of equipment being a techie, and I have a lot of professional experience setting up new machines too, so I know what I'm talking about when I say I can't tell the difference.
Even on these forums (which have a tendency to make very public even the most minor or rare issues) the stories of people getting dented, scratched, or dirty units are extremely rare. If it does happen to you, I am sure Apple will exchange it for another no problem.
Refurbished offer a GREAT value. Not only do you get a like-new computer but you save big money. The $1999 Core i5 model which just got updated is now $1529, saving you $470. If you think about it, that's almost like getting a free iPad!
Or if you get Apple Care for $169, you basically get a great iMac WITH 3 years of warranty and tech support coverage and you are STILL saving $300 over a "new" one with just 1 year of warranty and 90 days phone support.
Even the base model, retailing for $1199 not long ago is just $929 now, saving you $270.
I don't see why anyone [savvy enough to know about refurbs) would really buy a new mac (assuming a refurb of that same model is available).
And I am not a person who enjoys using second hand stuff personally. I have used some second hand products when they weren't important to me, or acquired some for family who needed deals. But me personally, I like my stuff new, especially my tech toys. I don't think of the Apple refurbs as second hand at all.