Iron Pipes will occasionally bug when they have more than 2 additional Pipes connected. This bug presents itself as the pipe visually showing it will allow items to pass through in a direction, but instead the items either bounce back the way they came or move off in one of the blocked directions.
Iron pipes can be Waterproofed.
Iron pipes can also be used as an extremely handy alternative to Obsidian Pipes for dealing with overflow. Place two iron pipes in a row, both pointing the same direction, and put the second one on top of the destination machine so it flows directly into it, such as a furnace. This way, when the furnace has, say, copper ore inside, but another ore comes through the pipe, it won't get "popped" out of the pipe and sit there to be picked up by you or despawn. Instead, it will turn around in the second iron pipe and head backwards into the first iron pipe... which will then send it forward again, repeating over and over until the furnace has finished with the copper ore and allows a new ore inside. A limit to how many blocks can be held in suspension like this has not yet been found.