Note: The Gitzo GM3550 6x Carbon Fiber Monopod has been replaced by the Gitzo GM3551. Enhancements include a belt clip on the strap and "the new aluminum Power Disc for a more secure locking of the head thanks to the additional locking screw. Removable rubber feet are now self-locking to avoid accidental loss."
Since the Gitzo GM3550 6x Carbon Fiber Monopod is very similar to the Gitzo GM5540 Monopod, I'm assigning the reading of the Gitzo GM5540 Monopod Review as a prerequisite for this review to avoiding saying many of the same things twice.
The Gitzo GM3550 6x Carbon Fiber Monopod shares most of the GM5540's great features including G-Locks, 6X Carbon Fiber Legs and great construction quality. Still, the differences are many.
Compared in the picture above are the Gitzo G1588 Monopod (top), Gitzo GM5540 Monopod (middle) and Gitzo GM3550 Monopod (bottom). The same monopods are shown below from a top view. The G1588 is on the left, the GM5540 is in the center and the GM3550 is positioned on the right.
While they are similarly-sized retracted (the GM3550 is 21.3", 54 cm), the GM3550 has one additional leg section (5 total) that allows it to extend much higher than the GM5540 - to a very-high 75.6" (192 cm). Unless you are really tall, this height would require steps, a ladder or another photographer height assistance tool to reach (remember - the camera and lens have a height of their own above the monopod height).
However, extending the narrowest (bottom) leg section to get this height results in more wiggle at the bottom leg lock than I prefer. With this section closed and locked, this is a very solid monopod. It handles the Canon EF 400mm f/2.8 L IS USM Lens and the Canon EF 600mm f/4 L IS USM Lens with ease. The GM3550 is rated to hold 36.9 lbs (18 kg) - significantly more than the larger-diameter Gitzo G1588 was rated.
The lower weight rating compared to the GM5540 is likely the result of a smaller-diameter leg design. You won't notice much difference in stability, but you may or may not like the thinner grip section. My personal preference is for the thicker grip - I can control the camera and lens slightly better with it. People with smaller hands (mine are medium-sized) will be better able to control the GM3550.
The Gitzo GM3550 6x Carbon Fiber Monopod is about 20% less expensive. It has a smaller top plate. At 1.4 lbs (600 g), it is lighter (compared to 1.9 lbs / 0.9 kg). I could be very satisfied to use this monopod for all of my needs.