Note: The Gitzo GM5540 6x Carbon Fiber Monopod has been replaced by the Gitzo GM5541. 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."
Do you get tired holding your camera and lens in use or in a ready-to-use position? Do you need assistance in holding your camera and lens steady? Do you shoot sports? Do you need a compact camera support? If you answered yes to any of these questions (and even if not), you may need a monopod.
Which monopod becomes the next question. You probably have invested a lot in the camera and lens you will be mounting on top of the monopod, so quality-construction should be a requirement for your selection - you don't want to risk having a monopod break. My initial monopod search resulted in a Gitzo G1588 Monopod (now discontinued) becoming part of my kit for many years. I liked it a lot, but when Gitzo introduced their 6x line of replacements, I upgraded to the Gitzo GM5540 6x Carbon Fiber Monopod.
While there are many changes between these models, the new leg locks - "G-Locks" - are the biggest real difference. The G1588 had twist-lock leg sections, but the G-Locks are smaller and much nicer to use. All G-Locks can be twisted loose at once and re-tightened in any order because of the new Anti-Rotation Leg (ALR) system. By design, vertical weight applied to the Gitzo GM5540 6x Carbon Fiber Monopod results in an even tighter lock. The 1.9 lb (0.9 kg) GM5540 is rated to support up to 55 lb (25 kg) - about double the G1588's rating of 26.5 lb (12 kg).
Twist-lock-type leg locks are my strong preference over flip locks for monopods. I like to slowly tighten the last leg section as I am lowering the camera to the ideal shooting height.
Carbon fiber is a great monopod and tripod construction material. It is light and vibration-absorbing - and doesn't transfer cold to your hands like aluminum does. Since there is less leg material in a monopod, the weight savings is not as great as in a tripod that has three times as many legs. But, it is still my preferred monopod leg construction material. The 6x carbon fiber design is new and reduces weight even more, but the additional weight loss is small.
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.
Another easily seen change is the top plate of the GM5540. It is now larger (compared to the G1588). A larger top plate provides a more solid platform for the camera/lens to mount to - it is more able to withstand torque. It also has another benefit for me. I use a Wimberley Quick Release Clamp on my monopod. The larger round top plate covers more of this rectangular-shaped clamp, making the monopod more comfortable to handle - there are fewer hard edges against my hand.
The top plate also acts as a wrench to allow the mounting stud to be reversed - both 1/4"-20 & 3/8"-16 studs are provided. A set-screw prevents unintentional rotation of top plate when tight.
As seen in the picture above, the GM5540 is shorter than the G1588 when retracted - 21.7" (55 cm) compared to 22.8" (58 cm). This doesn't seem like much (and it isn't), but it was a very nice change for me personally. I had been looking for a case for my G1588 since I first bought it, but never found the right one. The GM5540 now fits in a Gitzo GE10P Tripod Case with the Wimberley Clamp attached. This gives me an easy way to carry the monopod, protects the monopod from damage - and protects my gear (and car windows) from being damaged by the monopod. Sometimes it is the little things that make us happy.
The GM5540 also loses a small amount of height - it is now 63.8" (162 cm) compared to the G1588's 65.0" (165.1 cm). That is plenty of height for me (I'm 6') and my uses for the monopod - which are most frequently sports action.
The Gitzo GM5540 6x Carbon Fiber Monopod is a really solid monopod. It provides a wide, firm-but-padded grip that gives you great control over even the largest lenses including the Canon EF 400mm f/2.8 L IS USM Lens and the Canon EF 600mm f/4 L IS USM Lens. Build quality is very good overall.
Narrower and lighter monopods such as the Gitzo GM3550 Monopod offer lower prices and higher working heights, but I prefer the greater control and stability of the Gitzo GM5540 6x Carbon Fiber Monopod.