We an older Sevylor ST5656 sit-on-top on Craigslist, and decided to add it to our flotilla for novices/kids and were pleasantly surprised.
It's light and compact enough to travel with (we're taking it to Hawaii with us) and we're looking forward to using it in quiet water and in the surf.
It has little flex, and tracks well with the three attachable skegs. The deck is covered with Sevylor's fairly tough material, and the hull is pretty durable multi-layered PVC. The seat, while low, is easily adjustable, as are the foot rests.
The PVC air bladder is thicker in the older models, but you'd have to make some effort to get through the hull coverings.
The biggest drawback is the valves. Probably to save money rather than use the Boston valves Sevylor uses in almost everything else, the valves on this are overlarge beach toy valves, a simple tube with a soft plastic flap to hold air in. If you were to find one of these in fairly good shape, you would probably be smart to replace the valves with another kind of valve as soon as possible.