These are kettlebells, I believe the biggest one is an 88 pounder, then 70, then 53, 35, and last a 26 pounder.
They have way more variety than dumbells, because of the shape. And because you do some balistic lifts with them and it is a very thick handle your grip strength is severely taxed.
Now are they better for putting on mass than barbells and dumbells, NO WAY , you will never see a kettlebell only practitioner who is massive.
Another draw back is they don't lend themselves well to pectorial training, long considered a sign of athletic performance in this country, Every guy in the gym has been asked, or has asked, " how much ya' bench", If you were to say two 32 kilo kettlebells for ten sets of 5, people would look at you like you just said, " Hey, I think Osama bin Ladin is misunderstood, we should here his side of the story."
Because of the drawbacks of kettlebells most practitioners do cycles of freeweights and kettlebells and get the benifits of both, one of the best benifits is the extremely hard workout it puts on your core, your base of power, even a simple military press fires your core hard, because the weight hangs outside your arm and you have to use your core to balance it.
For me the #1 reason to do them is it is flat FUN, it is way cool to swing a kettlebell, toss it up and try to catch the handle again and do ballistic throws with it. As weird as it sounds I can't put the damn thing down, just sitting in front of the tv, I do some specific grip stuff because it is so fun to play with.
If your training has gotten stale pick one up, trust me it makes you feel like a kid again, when you picked up a rock in the field just to see if you could pick it up!!!!