Have to rate this for what it is: a kids toy to get them interested in paddling. A solid 10 in that respect, the kids love them. We've had them for a few weeks now and the kids are now using them as paddle boards; they are that stable.
My kids are 9 and 7 and on the smaller side but my 125lb niece was using it and having a great time, with a slightly wet butt from waves and riding lower. My 100 lb wife uses it easily and it has spurred us to buy kayaks ourselves. The kids easily climb back on them in the water. I have watched them intentionally flip them many times, flip them back over and jump back up. My wife said they are extremely stable even riding over boat wakes. She said to flip it it pretty much has to be intentional (accidents happen though). They are very durable, I dropped a rope through the front drain holes and tied it from the bottom and towed it behind my 30hp inflatable for some white water kayaking. My 16 and 18 year old nieces said they prefer it to tubing since you can lean in the corners to stay on. If jumping waves dragging them behind a boat didn't break them I'm not sure what will. I think they will last a long time.
The family the plays together stays together.