My son who boxes has gotten really strong using this product.

When I received this it didn't quite meet the expectations of the pictures. I immediately took it apart, because that's what I do. It has a polyurethane bushing that acts as the bearing when it rotates. This is perfectly acceptable for what it is, it is thick enough that you will never wear it out. The fitment of it on bushings is a bit sloppy, but after using it for some time this is a non issue as well. This is an item I was going to immediately send back after unboxing it, due to my first impressions of it. After using it and getting used to how it works I couldn't be any happier with this new piece of gear. I am an experienced weight lifter
If you are an experienced lifter is this going to build mass to your shoulders and arms.. No. But to use this as a warm up tool for the roator cuff muscles or a finisher to a shoulder or chest workout I think it works great for that. I also will use it while riding my concept2 exercise bike for a little extra upper body workout. For my use I purchased the 8lbs, it actually weighs in at 9.0 pounds (whatever). It will definitely give you a burn as finisher to a shoulder workout. I think it would be great tool for rehab a shoulder injury as well, and get the lighter weight one, I believe is 6#s. I think the 12 lbs would be great also, but for my needs I am good with the 8 lbs (9# lol)

I bought this for aerobic exercise, since I am currently without the use of one leg, and it was recommended by Nick Scott (a champion wheelchair bodybuilder). I was hoping to use it for steady-state cardio, but you'd have to be much stronger than I am to use this for twenty minutes straight. What works for me currently is alternating the 'Spinner' with knee-ins, Tabata-style.
This is a well-made machine. Simple, but effective. Nice bonus is the two open metal circles between the spin units. If you thread elastic bands through there and anchor them with your feet you can do a variety of curls and presses adjusting your grip as you lift (to focus more on pronation, e.g.).
Price seemed high to me, but I find I am getting a lot of use out of it, so probably worth the money.

Very sturdy, 12lb is no joke, it gets heavy fast. Probably a complete arm and shoulder workout by itself. Go in reverse in a bent over position or forward in a speed-bag position. I almost wish I would have got the lighter one, I feel the shoulders give out before the arms just due to the weight. Maybe a 10lb version would have been the golden ticket.

Okay so I am putting this to an odd use apparently.

I use it to pre exhaust sure, but mostly it sits on my desk in my work from home office -- and when I find myself getting bored/wanting food for entertainment/restless and wanting to pace while I process a thought I pick it up and work it until exhaustion. Might repeat a time or two.

The endorphin kick usually sorts me.

Very glad I opted for the 12 with this use case; and I ... kinda doubt most of us do any kind of competition with our muscles. Just pushing our limits for the joy of doings so; so who really cares if we're not at top efficiency.