I’m also about 3-4k into my smart arm setup but for a piece I use every workout it has quickly become an essential piece for me. Both the upper body and lower body movements feel great, and if you like tinkering you will be able to come up with a couple extra exercises that prime doesn’t advertise.
Definitely not an essential piece to perform leg extensions with the smart arm, but does help with sliding down the bench. Expensive for what it is, but really helps with “locking in” during leg extensions which is how me and my wife like to perform the movement.