The pros of using World War I aircraft were that they were very helpful in reconnaissance. They could fly over enemy lines and see the enemy's troop movements, and, after they were eventually armed with machine guns, they could provide an offensive advantage. 

The cons of using these planes were that they were very dangerous, and the average British pilot had a life expectancy of about 70 hours when flying above the Western Front (see Lawson and Lawson, cited below). The aircraft were too cramped to carry parachutes. In addition, it was very time consuming and difficult to construct planes during this era. A typical two-seat plane had more than 50,000 different parts and took 4,000 hours of labor to put together (see the Red Stone Rocket link below). Some planes of this era cost about $7,000 to build (at a time when the Model-T cost $400), so they were also very cost prohibitive.

Source:

Eric Lawson, Jane Lawson. The First Air Campaign: August 1914-November 1918 (Da Capo Press, 2002).

WWI airplanes were quite novel, for their day.  They flew quicker than observation balloons, and allowed the pilot a career alternative to fighting in the trenches in France.  Pilots could become celebrities overnight, such as Eddie Rickenbacker and the Red Baron.  Airplanes could carry a small bomb load, and once the machine gun was synchronized with the propeller, the plane became a viable fighting force.  The WWI airplane did have its drawbacks, however; the main one was that it was flown by an inexperienced pilot in most cases.  More than half of these prospective flyers were killed in training exercises.  The plane was poorly made by many standards--they were comprised of wood and canvas.  They could not carry large bomb loads; therefore, generals were more likely to use heavy artillery in land campaigns and use the plane as an observation tool that was an alternative to the observation balloons and blimps.