I bought one of these because I had a bunch of clients asking me about it and I wanted to be able to give them an intelligent answer.
Do I think it will give a man the upper body development their models depict? No, absolutely not.
Do I think it's "totally worthless"? No. Where I think it has some merit with respect to tennis is for scapular stabilization and strengthening of the rotator cuff muscles. There are certain positions you can put your arm in when you hold the weight where you're able to generate a stabilization demand on the rotators of the humerus. I think the oscillation effect combined with relatively low weight held in a stable position gives this device some potential for shoulder maintenance.
the ads are a riot and yes, can easily be viewed from a salacious point of view. Would I recommend this device in general? No. But if you have one I do think you can get value out of it in terms of helping your tennis game.
personal note: when I got this thing, I was about 9 months out from relatively serious rotator cuff surgery. Tore about a third of the subscap tendon off the humerus. Range of motion was about 80% of normal and even though I went through intensive physical therapy, strength still wasn't all the way back. After experimenting with this device, I noticed a definite and quick improvement in shoulder function. It allowed me to get after some small, deep muscles that are somewhat hard to access otherwise.
take it fwiw.