Comparing the A380 and 787 is crazy, totally different purposes with entirely different target markets. Closest Boeing product is the 747-400 series and derrivatives. The 747 has been a good seller for decades now (after some early difficulties) so obviously there was a market for such a large aircraft on long-range routes. It also was in a market without competition, something the A380 will change, but whether the A380 is a success will really depend on the market conditions for air travel in the next decade or so and whether it lives up to it's billing on cost/seat-mile. Boeing doesn't seem to have anything firm for a 747 replacement and is mostly looking at some additional upgrades to increase capacity on a design that is somewhat dated.
From what I've read, the A380 order committments to date are probably better than expected considering no US operator (except FedEx and UPS for cargo versions) has ordered it. You can bet if it lives up to billing that many airlines will seriously consider purchasing the A380 and cycling out their 747's. Good market for 747 conversions to cargo right now so a good time for airlines to make the change.
-Soda