I'm running 3240x1920. It really does make a pleasant difference. I sit just as close to this display as I did a single 1080p and Landscape eyefinity. I had so many problems getting landscape eyefinity to work well because most games don't scale well to it. For instance, playing NFS: Hot Pursuit, looks great, but if you want to see your pole position or speedometer or any information really, you'd have to turn your head to the extreme right or left, and take your eye off the road. Same goes for most FPS games.
Now at Portrait eyefinity, all those HUD stats are where they are supposed to be. And because you are sitting just a couple feet from a 40" super hi-res screen, you get just as immersed in the game as Landscape eyefinity, if not more. In a FPS game, it's pretty amazing that the size of the gun you are holding looks about the actual size. It feels like you really are in the game, not playing it on a screen. If you sat in front of a 40" 1080p TV, you'd get a headache from the screen door effect, counting pixels and looking at jaggies.
From my personal experience:
1080p gaming - standard
Landscape Eyefinity - Immersive
Portrait Eyefinity - Very Immersive, sometimes overwhelming
5x1 Portrait Eyefinity (I have not seen personaly in action) - I'd imagine is something like playing video games on an IMAX while sitting in the front row.