Lately, being a broke college student, I have been buying used games more often than I usually do. Personally, while I do understand that it does harm the industry a bit, economically your first concern should be yourself. If you have enough disposable income to buy a new game and are willing to do so than that's great. However, if you decided to save a little and buy used I'm not going to lose any sleep at night.
I have heard the idiotic argument that buying used games is just as bad as piracy. While the situation of used games as a whole might be more harmful than piracy this is because the act of buying used games is more common. If I said that more people die from smoking than they do from homicide that doesn't mean it's better to kill someone than it is to give them a cigarette. Unlike piracy buying used games requires a pre-existing copy that someone has bought new at some point in time. While no money goes to the developer it helps support the retailer, which in part provides an outlet to sell new games, including more obscure ones that other outlets like Wal-mart or Target would not stock.
And of course there's the argument that used games help sell sequels. Let me provide an example. I am interested in Uncharted: Drake's Fortune but I'm unfamiliar with the franchise and so am not willing to pay a large amount of money. I buy the game used for cheap and greatly enjoy it. Uncharted 2 comes out. I am now familiar with the series and know I enjoy it and so I am far more willing to buy this installment for $60 new. It's a stretch but used games in some small way help developers.
Do you buy used games and how do you feel about it?