While it's not a direct benefit of Eyefinity, one of the greatest benefits I find of a multi-monitor system is the ability to create a customized media hub. Most monitors will let you attach more than one device, so even with a single monitor system it's possible to hook up both your PC (DVI) and your PS3 (HDMI). However, I find that the multitude of connections available in a multi-monitor configuration opens up a wealth of possibilities.
These benefits shouldn't be discounted when you are considering and investment in an Eyefinity system. And, the additional options available with moving to a five or six monitor setup should be considered as well.
I do need to point out one issue with multi-tasking on DisplayPort connections. DP is an active connection, and communication runs both ways. If you disconnect the cable, Windows will know. If you have an Eyefinity grouping with that monitor, it will be broken. This could prove problematic if you are in-game and change monitor inputs to multi-task for some reason. You may be able to get around this by using the DP>DVI adapters, but I'm not sure.
I was surprised the first time I experienced this on the Dell P2210H monitors, as it hadn't occurred with the Dell U2410's. My best guess is that the Picture-In-Picture feature on the U2410 keeps all the connections active all the time. There may be another reason, but do keep this in mind.
Moving to portrait does have some drawbacks, but 3x1-L and 3x2 have theirs as well. The portrait configuration requires you to look at certain documents or programs in a different light. It's helped me see the WSGF as many other members do. Most other websites are rather narrow as well (fitting within the 1080px) and browsing is actually improved with the longer length.
Multiple spreadsheets or PowerPoint style documents fit better one above the other on a portrait display, versus side by side on a landscape. Dreamweaver offers a similar benefit with allowing code and page preview to both show extensively on over the other. The only real productivity issue I've found is Photoshop, but I'm learning to live with that. Finally, DVDs and Netflix streaming still plays at full resolution within the 1080px width of the monitor.
My main issue with 3x1 is "tennis neck" from looking left and right. On both 3x1 and 3x2 I lost my cursor quite a bit. The landscape offering seems to be better for keeping track of the cursor. Make no mistake, all of the configurations are massive surfaces and can sometimes be a bit much to deal with in a Windows desktop.
My big complaint with 3x2 is that neither set of monitors was at the right height. If you centered the overall monitor matrix to the center of your vision, then you were looking at the center bezels. Do to the fact that I have a shallow desk (which backs into a corner), I was sitting too close to the installation. I was craning my neck up or down to see each row of monitors. While backup and focusing on the overall image is fine for gaming, it put productivity tasks a bit out of eyesight.
Given all of that, I was more excited about the possibility of the 5x1-Portrait as it would reduce the overall height and eliminate the center bezel. I found that I could center the monitors and see the top and bottom edges just with moving my eyes (rather than my whole head). Games currently play on the center three monitors (until ATI releases the 5x1-P driver update), and that is where non-Eyefinity games will continue to play in the future. The 5x1-P setup allows for the outer items to multi-task while gaming on the center three.
I set up the five monitors in a similar manner as the 3x2. The Eyefinity6 is connected to all the monitors. The work laptop is connected to the left monitor via VGA, and the Mac Pro is connected to the other four monitors via DVI. Gaming on the three center monitors (and disabling the outer two monitors) allows the outside monitors to be used without breaking the Eyefinity group. In the last shot below, you can see Guild Wars running on the center three monitors while I have the Guild Wars Wiki up on the Mac Pro on the right-hand monitor.
If you have multiple computers, this configuration allows for true multitasking while gaming (with multiple mice and keyboards) without the need for Alt-Tabbing from inside the game.