I'm going to shamelessly bring up this topic again. There is no definitive answer I've seen.

First thing first: This is referring to a vaporizer, not any method that burns the plant as that would be too harsh and possibly cause infections.

Biology is the only thing that matters. The mucus membrane in your nose is the key point. When anything brushes up against them they get absorbed: bacteria, viruses, and THC. It's the same principle in your lungs, except your lungs are more adapted to dealing with the particles in a gas. Your nose isn't designed to do that nearly as well, being an extremity and vulnerable to the elements. Even if it isn't the most efficient way to collect THC, at the very worst it will still increase the intake by a smidgen, though I believe it should be significantly high.

You would also absorb more of the THC, due to the triple filtration process (In through the nose, into the lungs, and out the nose again). The proximity of the nose to the brain would decrease the time it takes to feel it, and the quality would be much more significant.

You might want to sterilize the piece before sucking on it again though.

As for pipe/bong smokers, I wouldn't recommend inhaling through the nose. Your mouth is a lot tougher than your nose. Remember the harshness the first time you hit a bong? The other particles will fuck your nose up, and also possibly contain bacteria that will just directly enter your system.

Anyone else?