I have a Co-Motion "Co-Pilot Road" ( a single bike, not a tandem, despite the name) with S&S couplers that I've had for six years and have taken to Europe and Asia several times.
The couplers have absolutely NO effect on the "feel" of the bike and are completely transparent as to ride quality. If you don't look at them you have no idea there is anything unusual in the frame.
They are extremely durable. The ends are joined by interlocking tapered teeth and held in place by a threaded coupling nut. They are self-adjusting for wear and made of such hard material that wear isn't a problem. They will be the last thing standing when the rest of the bike is scrap.
Corrosion also is no problem. The most common couplings for steel frames are made of hardened stainless steel and the couplings for Ti frames made of Ti with a stainless steel coupling nut. S&S can supply less expensive Cr-Mo couplings to OEM builders but these are unusual and wouldn't be any more corrosion prone than any steel frame. Again, any of the couplings will easily outlast the rest of the bike.
There are two downsides to them. 1) They are expensive, adding $200 to $400 to the cost of the bike and repainting is required if they are retrofitted to an existing frame. 2) They add about 200 grams to the frame so the weight weenies will be troubled by that.
I highly recommend them.