I've heard all the things on both sides of this issue, whether or not violent games lead to real violence or not, and I've never really come down on one side or the other of that debate, but I have a bit more insight into it now.
I have owned a PS2 for a few years and recently acquired a PS3, and most of my collection has been sports games and a few of the many titles set in WWII. The WWII titles were some of the Medal of Honor ones and a couple of the older Call of Duty titles.
I don't get tired of those games because I've always been interested in WWII since I had to do my own research to find out about the action my grandfathers were involved in. Neither of them would ever talk about it, so I took it upon myself to look things up and do my homework. My paternal grandfather was in the 101st Airborne and was in some pretty harrowing fights and my other grandfather was a diesel mechanic who worked on ships, so he wasn't in the thick of it.
So with all that in mind, I recently picked up Call of Duty World at War and I've been playing it on my down time this week when I finish deliveries in the evenings. Those who have played or seen this game know that it is a bit more graphic than previous installments in that series. Having never been in combat myself (never even been in the military) I feel that this comes closer than other military games I've played to capturing some of what it must be like. (If you discount the whole go back to the last checkpoint if you die thing)
What kind of effect has it had on me so far? It's hard to tell, but it hasn't made me more violent. If anything, by showing in a small way the utter brutality of combat, it has given me an increased appreciation for those who have been through it. I have always believed that our military people should be held in high esteem for their sacrifice, but this game has driven that point home more than previous titles that weren't quite as graphic. That doesn't mean that I enjoy the other ones less; in fact some of them are better for a momentary diversion than this one is because, at least for me, it is somewhat emotionally draining. Maybe I think too much about things, and maybe a lot of other people just view it as another shoot em up with better graphics, but this had the opposite effect on me that some say it should have. It didn't make me more violent, but even less so if that could be possible. I guess it really depends on the player and that person's personality.