I’ve used it as a rehab exercise, both light and heavy-ish. The problem with it is a physics thing, due to the the angle the knee is at and just as importantly where the resistance is. With the knee bent that way, the knee cap is pushed hard into the knee joint, causing a lot of grinding on the bottom surface of the kneecap. Then when you get to the top, same thing. At the same time at the start and finish, the meniscuses (menisci?) are also compressed. Use too much weight and/or throw the weight around too much, and you’ve got some problems waiting to happen. Besides the potential for an acute injury like that, doing the exercise over time will wear down the structures.
It can be quite good for rehab. It’ll isolate specific muscles depending on how far you go with it. One of the most common problems following ACL reconstruction is atrophy of the VMO (vastus medialius, or the teardrop shaped muscle of the thigh at the kneecap towards the inside). Going to that full extension targets it; the reason why that muscle atrophies is because ACL people have a hard time locking out their knee. If I didn’t use an isometric machine for it (using hydraulic resistance where it’s even resistance all the way through) and had to use a traditional one, I’d make sure they weren’t throwing the weight around, were starting at a better angle than the fully bent one, and they weren’t slamming their knee into full extension. I didn’t use the traditional machine with everyone, but did it when I really needed to. I used a rowing machine far more often, as the motion and range of motion are the same, but you don’t have all that stress at that angle.
Sorry to bore you guys