What he said. Cooler Master Hyper 212 EVO is also very good.
Yup, almost as good as the Noctua for less than half the price.
The Noctua's split tower design makes quite a bit of sense since it distributes heatpipes more evenly across the fins, gives the pipes more exposure and the mid-mount fan re-mixes air so airflow that had little contact with fins through the first tower will at least contribute to cooling the rest of the airflow before going out into the second tower. With 212-style towers, heat pipes are lumped together at either end of the fin stack which reduces the pipe's exposure to "fresh" air and leaves the middle of the fins under-used.
We will probably see more split-tower type designs in the future, it is the logical next step for that sort of design.
The next step beyond that might be liquid closed-loop within the tower itself.