The S-16 is a dynamite shotgun. It's a 16ga made on a 20ga frame. The dynamics are just excellent, and it was quite a popular and desirable shotgun.
The vent rib brings a premium since only about 30% of the guns were so equipped. They are also usually found with full choke barrels.
Since there is no 16ga event at skeet, it's common to find examples with little or no mechanical wear, but with much evidence of carry and use.
The weakness is in the wood, escecially with cracked forends. They used light weight wood for these to keep the weight down. A crack isn't a disaster, but it should be attended to promptly and I personally use nothing hotter than a standard 1oz field load in my S-16.
No reason to stay away from a S-16, and every reason to own and enjoy one. They have been skyrocketing in price, at least for high condition examples.