I have triple screen eyefinity, but mostly play Starcraft II so I use just the middle screen for that.
When I play FPS games, I think of it's effect as very nice. When you have eyefinity, it's like looking out of your car where you have the front windshield and two side windows so you get a full view. Yeah, you mostly look out the front window where the action is, but nice to have the side windows (and side mirrors) for peripheral and when passing etc. and the A-pillars are there to block your view just like the bezels of a monitor. But not using eyefinity would be like blocking out the side windows of your car - you could still drive, but it would suck not having peripheral vision.