Bi-directional path tracing

There hasn’t been a discussion about this since I began to follow cycles more closely.
I’ve heard of bi-directional raytracing(?) but don’t yet know the non-speed downside.

What is your opinion of the viability of Blender using it? There may be technical, legal or common sense issues about it I know but it sounds like an interesting topic; if not the holy grail of development solutions.

There’s a thread here that has some interestings bits

Excellent! That answered a lot of questions. Thank you very much.