I've seen that other automation systems that use power-line communication like Insteon and UPB also have their share of problems with power-line noise and signal suckers. It's all very useful when it works, but power lines simply weren't made to be data buses. However, I also think that the current X10 RF implementation is nice as a supplement to PLC but is not nearly robust enough to build an entire home installation around, even in a small house like mine. The transmission distance needs to be better, it needs to detect collisions and retransmit, we need repeaters that don't compete with each other, we need at least status query, extended dim, and some manner of scene support, and we need some manner of privacy or encryption. Some of these are doable without breaking compatibility with existing devices, but not all are. If they're going to go RF, it needs to be something that can compete with Z-Wave. Just my opinion; feel free to disregard as desired.
EDIT: Clarification: As a supplementary system, I can forgive/cope with these weaknesses and mitigate the issues that result. As the core of a home automation installation, it would be unacceptable.
While on the topic of interference, I'll note that when there is power-line interference, there is usually something you can do about it since you have physical access to the wiring that is used as the bus. With RF, if a neighbor or nearby business happens to have some device that spews out strong noise at the same frequency that your RF gear uses, you don't have any options short of building a Faraday cage around your house.