Full Suspension bikes range from about 100mm(4 inches) of travel for an endurance racing bike, to about 8, 9, 10 inches of travel for Downhill Racing Bikes. Pedalling around more than six inches of travel can be a bit of a chore. If you're looking for an efficient ride, limit it to that 150-160mm mark. If you're looking to race Cross Country(XC), 100mm is the most practical.
Full suspension tends to be more fun at higher speeds than a hardtail because you float over the rough stuff a little easier. It's also much more forgiving on the body.
The cons are the weight, which with a high end bike can be negligable, and technical ability at low speed. It takes a little umph out of your bunnyhop.