Actually, the level of immersion that you gain by adding the extra resolution is something that is very hard to describe until you see it for yourself. You won't feel comfortable going back to a single screen once you have played with 3 monitors. Some games will feel claustrophobic, specially any FPS.
Not all games are optimized for multiple monitors and the hardware requirements are considerably higher; you need at least a 7850 with 2gb of video memory and a phenon 1090t or equivalent. The following video is recorded with fraps in eyefinity and you can see how the image looks more clearly:
Back on topic: I finished the DLC "Vergil's Demise". I found it a little too short but it was fun... the story was a big cliche though.