The Thermalright Ultra 120 Extreme heatsink has a surface roughness of approximately 16 microinches, which is considered excellent. The nickel plated copper base of the Ultra-120 Extreme is machined smooth, but it's not entirely flat. The base is perfectly flat in the axis parallel with the heatpipes, and noticeably convex in the axis perpendicular to the heatpipes. You can see the curvature in the image below. A known straight edge was held to the base in front of uniform light source.

As popular as the Thermalright Ultra-120 Extreme is, this curious base 'treatment' by the manufacturer has spawned a cottage industry amongst CPU heatsink retailers for pre-lapped heatsinks.

360° View - Thermalright Ultra-120 Extreme Information on Frostytech's test methodology is available here.
The Thermalright Ultra-120 Extreme heatsink is built to work with any 120x120x25mm fan that spins in the region of 1400-1600RPM. Rubber anti-vibration strips applied to the frame keep metal fins from rattling. Each nickel plated aluminum fin measures 0.45mm thick, and is spaced 1.6mm apart from the next. Over all there is 126 x 107mm worth of cooling fin (11mm less hight than the Ultra-120 version). The six nickel plated copper heatpipes are 6mm in diameter, and extend from the base up through all the fins. All joints are soldered. Only 63 mm thick, the Ultra-120 Extreme stands 159 mm tall, with the first fin elevated 45mm above the CPU base. Heatpipes are positioned side to side at the base to ensure heat is quickly conducted up and to the fins. The base is capped off with a 50mm wide aluminum section that provides rigidity and strength for the CPU clamping brackets.
Looking from the top down, we can see how Thermalright have modified the aluminum cooling fins on the Ultra-120 Extreme to create winglets at each corner. The leading and trailing edges are alternatively bent 15° up and down. When a 120mm fan is installed, a 5mm plenum is created behind it, owing to the outwards curvature of the fins. The six 6mm diameter copper heat pipes are positioned towards the outer edge of the cooling fins, in the vicinity of the highest airflow from the fan. The nickel plated copper base measures 40x42mm in size and is soldered to the aluminum cap the mounting brackets attach to. The copper averages 2.25mm thick over the heat pipes. As you're already aware, the base is machined smooth, flat in one axis and significantly convex in the other.