UAVs are extremely popular at the moment, mainly because of the success of aircraft like the Predator and Global Hawk in providing persistent surveillance of areas below via a steady stream of real time video. UAVs have been around since the 1960s, but it was only with the introduction of satellite communications, and cheap, reliable digital video technology, that the UAV became a very valuable battlefield tool. Four decades of improvements in flight control computers and software, and aircraft construction, helped as well. More efficient and reliable engines made it possible for UAVs to stay in the air for up to 40 hours (or more.) GPS satellite navigation played a role as well, solving many problems associated with operating an unmanned aircraft. 

But there are problems. The attrition rate of UAVs is higher than expected. The Predator force loses about 12 percent of its aircraft to accidents and enemy action each year. For manned aircraft, the annual attrition rate is less than one percent. Smaller UAVs have even higher attrition rates, especially the micro (under ten pound) ones, because they are more vulnerable to the weather (especially strong winds.) Using GPS for guidance allows the UAVs to fly in all sorts of bad weather, but without a pilot on board, it has proved difficult to detect dangerous conditions, like ice build up on the wings in cold, wet weather. Tricky winds, even for the larger UAVs, has caused unexpected losses. So the UAV designers are still learning how to equip their pilotless aircraft to deal with unexpected situations. In addition, there are the losses from sending the UAVs on missions that would be considered too (or at least very) dangerous for piloted aircraft. 

The higher attrition rate for larger UAVs (like the Predator, which is the size and weight of small, two seat, manned aircraft, or Global Hawk, which is the size of a multi-seat business jet) diminishes the apparent savings. A Predator costs nine million dollars, while an aircraft that could do the same work (several manned recon aircraft, actually) would cost about the same when you included the cost of crews for twin engine propeller driven aircraft that are actually used for Predator type recon missions. The manned aircraft, however, could not be used as aggressively as the unmanned UAVs and would tie up a lot more people and airfield resources. 

From a military point of view, Predator has other advantages. For example, Predator puts more recon capability into a combat zone at less cost in terms of people and supplies. Never forget the importance of logistics. Even though Predators are going down at several times the rate of equivalent manned aircraft, they are still cheaper to support in a combat zone. And you are not losing pilots, which is a major plus (especially for the pilots and their next of kin.)

The attrition rate for these new UAVs was twice what the engineers had predicted. New technology often lacks the reliability and predictability of tried and true stuff. This is what worries the proponents of the next generation of UAVs, the UCAVs (Unmanned Combat Aerial Vehicles.). These will all be bigger than Predator, since they are built to carry a ton or more of bombs and missiles (Predator weighs about a ton on takeoff). UCAVs will fly more often under the control of onboard software, not a human pilot on the ground somewhere. The navy plans to operate UCAVs from aircraft carriers. Carrier landings are tricky, but given the current state of software and sensor technology, its certainly possible to build a UCAV that can do it. Actually, the navy has already conducted experiments, and it appears that current technology can bring a UCAV down for a carrier landing. But whether this can be done time and again, with a lower accident rate than manned aircraft, remains to be seen. Based on past experience with UAVs, the navy can expect to lose a lot of UCAVs, and have their carriers sustain some additional damage in the process.

Taking several, perhaps a dozen or more, UCAVs along on a bombing mission is another bit of unexplored territory. Such formations are common when the enemy still has intact air defenses. The strike package of aircraft includes warplanes that specialize in deceiving, or destroying enemy radars and missiles. In theory, UCAVs can handle some of this SEAD (Suppression of Enemy Air Defenses). In practice, theres going to be a lot of trial and error. Same with using UCAVs to fight other aircraft. While successful tests of this concept were held as long ago as the 1970s, no one has yet tried it on a real opponent. More trial and error. 

UAVs and UCAVs are the future, but they are not a risk free future, or one that is as cheap as some now think.