I recently added a 30" LG Monitor to my 3-monitor setup. Originally, I had intended to replace my 24" Viewsonic widescreen, but when I started considering it seriously, I began to see the utility in having a forth monitor, instead of just a larger third.
I was initially unsure of how I would accomplish this, but I had seen a 3M Monitor Arm in Office Depot, so I thought I'd buy it and try it out. Unfortunately, the 3M arm didn't have the lift that I really wanted, so I returned that arm and started searching online for other options. I came across the Ergotron Single Desk Mount Arm and the Ergotron Dual LX Monitor Arm. My thought was that I'd be able to purchase a dual arm, and mount both monitors on the left side of my desk on the arms, one above the other.
Once the dual arm arrived, I was able to get it setup in about 30 minutes. I was somewhat surprised at the ease of the setup. I had to unmount the other monitors from their stands, attach the monitor arms, secure the base to my desk with the desk clamp, and route the wires through the arms. The hardest part was removing the stands from the existing arms, and that wasn't very difficult. I did have to adjust the tension of each arm because of the weight of the monitors. All in all, I was very impressed with the ease of the setup process.
After a day of using the arms, I was so impressed that I ordered a second set for the monitor on the right side of my desk, and the Mx Desk Mount LCD Arm for the center 30" panel. The setup of the 30" monitor was a little more difficult than the smaller panels, but that's largely a function of the size and weight of the display. The desk clamp for the MX arm is larger than the other, and seemed very secure to me. The MX Arm is very solidly constructed, and has no problems whatsoever keeping the large display in place. I wouldn't attempt to use it with a smaller display, it really is designed for heavy displays, and the smaller arms do a fine job.
The cable management system of the arms is very well designed, and actually seems to work. I upgraded all of my DVI cables and power cables during this process to either 10 or 15ft so that the monitors positions could be easily adjusted, so the cables are rather large and inflexible, yet the cable management system still worked fine.
While I originally wanted the arms only to allow the two displays to be elevated, as soon as I got them installed I realized that I had needed these arms for some time, I just didn't know I needed them. For starters, they make dusting my desk significantly easier, as the monitors can be easily repositioned out of the way. Additionally, the appearance of my desk is cleaner than it was before, with the arms really improving the visual aesthetics of my work area; it looks much cooler and more high-tech now, and that counts for something. The arms have also made it easier to reconfigure the display configuration for specific situations. I'll occasionally shift one of the monitors to landscape view, and the arms allow any of the monitors, even the 30" display, to be transitioned.
From a productivity standpoint, I've got 4,096,000 reasons to be happy with this upgrade. The 2560x1600 resolution of the large display allows me to display either 2 or 4 files, plus the filesystem tree, in my IDE simultaneously. This has greatly enhanced my coding productivity. WIth the other three display, two 1600x1200, and a 1680x1050 display, I have a total of 9.7 million pixels to work with. In the upper left I usually keep my email, iTunes, and my project management software open. On the right hand side I usually have a web browser open with the stuff I'm working on. In the lower left I usually have a bunch of SSH windows open, tailing files, running irb, cucumber and autospec. In the center I'll keep my IDE or anything I'm actively working on. This allows me to keep everything I'm currently working on open and on top, so that I can reference things without having to move or change any windows. In particular, it's nice having autospec always running on my project so I can glance over and see any regressions.
All in all, I'm extremely happy with the monitor arms. I estimate the upgrade cost about $600, and my only regret is that I didn't make the investment sooner.