Construction Foam is a quick and effective construction material for defending against mobs. Players are able to break the foam before it hardens in no time. However, broken foam cannot be harvested.
After it's placed, it will eventually turn into a solid block, which is explosion resistant and paintable. With an explosion resistance of 60, it will resist TNT, Industrial TNT, and Creepers, but Reinforced Stone is a better choice for protecting against things like nuclear reactor explosions and lasers.
Wall blocks harden at varying rates. Foam hardens at 0.6% per 25 seconds, per light level. Foam placed in total darkness will never solidify. Right-clicking on foam with a block of sand will cause the foam to harden instantly, but the block of sand will be consumed in the process.
Players, mobs, and animals can walk through foam until it fully hardens (although the risk of suffocation or getting stuck in a new wall is high). To move through CF blocks the player needs to be sprinting, and being in contact with a CF block at "head" level will deal half a heart of damage to the player per second (unarmored).
As of 1.70, using a CF sprayer will allow you to cover placed IC2 cables, creating a block that functions as a EU cable but looks like a CF wall. This can be quite messy to clean up as it will do the standard CF-everywhere spray pattern. When a "CF-cable" block is destroyed, it drops the cable it was created with. (Will not work with just a block-in-hand method)
You can spray CF with the CF Sprayer too. Place Construction Foam in a Compressor to create CF Pellets. Load the CF Pellets into the CF Sprayer and you're good to go. The sprayer's capacity can be increased with the use of a CF Backpack.
Currently a single pellet will place a glob of 13 blocks of foam. Using scaffolding will allow you to arrange those 13 blocks into desired shapes, the most common being a straight line. See the CF Sprayer page for more details.
Note: This will leave behind an empty bucket. You can alternatively use a Water Cell in lieu of a bucket of water.