I bought these after experiencing the magic of monitor arms at work, and although they definitely do the job and I'm happy with them, I just can't not think about how the ones at work are better in most ways. That's why I've rated these 4 rather than 5.
Despite appearances, only the second half of the arms closer to the monitor have the full range of motion in all directions – the half closer to the base does not move vertically and has a limited degree of rotation. My work ones go the whole 9 yards with full rotation and easily removable backplates for the monitors that just slot in/out of the clip at the end of the arms so they're easy to clip off, screw onto the monitor and re-mount – the ones with these are fixed onto the arm unless you do some creative unscrewing and re-screwing of the head piece.
They do the job and with careful setup and wrestling with the arms I managed to get my two 24" monitors sitting nicely next to each other – can't imagine being able to do that with anything larger though without a big empty gap between the two. The two arms being connected to one base is more limiting than anything. If you're interested in this, I'd recommend buying two separate arms instead if you can afford it, as it will give you much more freedom with how you're able to arrange your monitors on your desk.
Finally, make sure your desk has enough space for the HUGE clamp you'll need to install at the back! It's about 10×10cm so you'll need 10cm of space free from the far end of the desk. I had to go with the drill-a-hole-and-use-the-grommet option instead because I had supporting beam in the way of being able to fasten the clamp.