We have a Co-Motion tandem with S&S couplers. According to Co-Motion, they actually make the frame stronger than a non coupled frame. We travel with the bike several times a year, and disassemble and reassemble it in about 60-90 minutes each. They are very common on tandems because otherwise the bike won't breakdown small enough for the airlines.

The ti couplers in a ti frame combination works very well. You'll save money having your new ti bike built with the couplers, but it is usually possible to have it retrofitted. I started the process of having one of my ti bikes (Carl Strong) retrofitted with S&S couplers at Bilenky in Philadelphia. Unfortunately my top tube was 1 inch and the smallest couplers are 1-1/4 in diameter.