You use the hot water at taps the way you would if it came from a separate water heater. But you also draw the water to an exchanger that transfers heat into a stream of air blown into delivery ducts. By the time the exhaust winds through the coil, most of its heat is in the water where you can use it. To decide if a new high-efficiency system is worth the investment, you need to know the efficiency of your existing system, about how much you spend annually on fuel, and the cost of a new system. Products, efficiencies and installations vary so much that there are no average figures to go by.
Installation is about the same in many circumstances.
But it can be substantially lower than typical costs if you don't have to build a masonry flue. Also bear in mind that many utility companies offer rebates on high-efficiency units. But money isn't the only issue. A new condensing unit can allow you to dismantle an old chimney that's in the way of a remodeling project. It also will release far fewer carbon dioxide emissions than an older furnace.