I use this foot along with RRS LCF-50B swapping between the two. The CP-51 primary advantage is lower profile which provides better balance mounted to a ball head, but more importantly allows proper balance on a compact gimbal like the jobu Jr3. Mounted in the Jr3, the CG is below the pivot point which allows the setup to return to level when you let go. With the RRS foot the lens is slightly top heavy and tends to drop forward of aft unless you apply some drag. The RRS foot works with light drag applied, but the CP-51 has the edge here. As a handle, I can grip it comfortable with bare hands, but with thin gloves, the gap is marginally too small. The RRS is much better here with gloves and a bit more comfortable overall in handling. When shooting handheld, the CP-51 foot resting on the palm of my left hand is much more comfortable as the RRS foot has cutouts that irritate after some time. Both feet are well made with the RRS being slightly longer. Both fit my RRS lever clamps well. the CP-51 shows a bit more flex in my setup allowing more vibration than the RRS which has the edge in this reguard. The hood reversed interferes with the CP-51 foot slightly, in that I have to apply pressure to get the hood to fit properly. This is a moderate annoyance. Overall, I generally like the RRS foot slightly better for general use. If it saw nothing but use on a gimbal and minimal handling, maybe the CP-51 is a better choice. A long lens support would help the flex.

Advantages: Light.
Compact.
Perfect balance for a compact gimbal.
Well made.
Less expensive.
Comfortable for hand held shooting.

Disadvantages: Flexes more than the competition.
Gap a bit tight for gloves but good bare handed.
Hood reversed does not fit easily.
Less comfortable/easy handling as a handle.

I wonder if we can attach a monopod on it when shooting sports.

Extremely light weight and well built.

Advantages: Light weight and low profile

Lighter and lower profile replacement foot. Critical when using with a sidekick to keep lens weight centered over the tripod. Still not bad to use as a carrying handle but not quite as easy as original for that purpose. I use with a canon 500/4. Probably not worth it if you are mostly using handheld but great if you mostly use on a tripod. A bit expensive which is why 4 stars.