I've tried a few of the sets with smaller balls inside them (so not the solid ones, if that is what you mean), and I enjoyed using them. Over several months I tried to use them or do kegel exercises on my own without them for 10-20 minutes a day, and it made a dramatic difference for me. One of the best things I ever did. I am much tighter, I noticed that sex felt dramatically better (and still does), and my orgasms are easier to reach and more intense. And as an added bonus, I can comfortably wait much longer before needing to use a bathroom (great for traveling and day trips).
I don't use them anymore since I don't need them, and they actually became painful to insert and take out because I am so tight (with 1" balls). I would suggest comparing ball sizes because it will become important as you get tighter. Large ben wa balls will get uncomfortable quickly, and some reviews even say they're uncomfortable from the start.
My thought is that they are great for beginners to kegel exercises because it helps you learn how to contract the muscle, and for some women it is a fun incentive to make them do it more.
However, the mystique surrounding them of possibly causing orgasms (in my opinion) is likely unfounded. I don't think I have read any reviews of ben wa balls on EF that say this- just that they are pleasurable and fun to use. So they are a great way to step into kegel exercises and they are a fun thing to use when you're doing things around the house (or out in public if you're feeling daring), and I think that is all that can be expected of them.
For me, I never found a set of balls had to keep in; the pleasure comes from the balls moving from side to side, and the smaller balls within them moving around during use. The more abruptly I move (like suddenly moving forward or jumping up and down), the more the inner balls more and the better they feel. It may be different for everyone, but the type of pleasure I experienced with them was just a feeling of being mildly teased, and more intensely if I tightened my muscles around them.
Sorry for the essay, but I hope that helped!