I agree with polaz, and I'd like to add the following
Advantages:
1. Increased efficiency (less fuel costs)
2. Less required cooling water
3. Better adaptability to load fluctuations
4. Although they are slower to start up than stand-alone gas turbine cycles, they're actually much faster than steam turbine cycles
Disadvantages:
1. Higher capital cost in the majority of cases (sometimes it can be actually cheaper to use a combined cycle than to use a steam cycle for the same capacity)
2. Adds more complexity to the system operation
3. Trouble in the first plant (gas turbine part) causes the entire plant to shutdown