I picked up a JM 336CS (checkered walnut stock, 336W = birch stock) recently for $350. It is in fantastic condition, 95%+. Another member here picked up a similar rifle a month or so back for $375. My point is that deals are out there if you are patient. I personally couldn't stomach paying $350-$400 dollars for a used rifle when the same model, brand new, is $370 at Walmart. I know the JM's are more desirable but they aren't rare (at least the W model isn't).
Now if you come across a rifle in good shape and for a price you don't mind paying, go ahead and buy it. Of course if it isn't what you want, you'll probably regret it. I know I'm glad I waited to find the one I wanted, and I passed on a good deal of $350 336Ws while I was searching.