I have had 2 distinct experiences in two different homes where X-10 did not work for me. In both cases I ended up using UPB components, which are more expensive, but much more robust than X-10. I originally suspected that my X-10 troubles were due to excessive noise in the electrical system (computers and fluorescent lighting).

Just recently I realized that both failed installations were in bedrooms and in checking further, sure enough, both bedroom circuits were on arc fault breakers. As several others have pointed out, it appears that the X-10 signalling is filtered out by these breakers. In both cases, I found that UPB components (which use an entirely different signalling methodology) worked fine without the use of filters or couplers.

The same X-10 components that failed to work at all on arc fault breaker circuits seem to work fine in other areas of the home that are on standard circuit breakers.