The NH-L9i is a small cooler that's built for small places (but nothing's stopping you from putting it in a full ATX case either!). It's 95x95mm square and 1.5" height make it hardly any larger than the CPU area on your motherboard, and no taller than the average installed RAM stick. Due to the cooler's compact size it probably won't be your cooler of choice if you're planning on doing extreme over-clocking or running high wattage CPU's. Noctua recommends keeping it on a CPU with a TDP of 65W or less, or 95W or less in a well ventilated case. More often than not, if you're putting the NH-L9i in a case so small that it requires a small cooler like this, then you probably aren't over-clocking or doing extreme gaming on that system to begin with.
The heatsink portion of the NH-L9i is all but swallowed up by the fan on top. Both the heatsink and the fan are of low profile designs. The heatsink is only 23mm tall without the fan installed, and increases to about 37mm with the fan mounted.
The fan that's included with the NH-L9i is a Noctua branded fan and thus has all the minute attention to details that all Noctua fans have. To start, each corner of the fan has a small rubber pad surrounding the screw hole (both on the top and bottom facing sides). The pad will help absorb vibrational noise when the fan is screwed tightly to the CPU cooler. I like the integrated pad design that Noctua uses on a lot of their newer fans because I can still use traditional metal mounting screws with it. Most manufacturers will ship their fans with annoying rubber mounts to serve the same purpose, but the rubber mounts, which are an alternative to using metal screws, don't allow you to install the fan in as many places that a traditional screw would.
Look closer at the blades and you'll see a few groves near the end of each blade. These are what Noctua dubs as "Flow Acceleration Channels" and they're supposed to speed up airflow near the tips of the blades which is said to increase efficiency and lower noise. If you follow the end of fan blades outwards you'll notice that the air inlet area that surrounds the fan blades is also unique looking; it has a stepped design. This is another unique touch from Noctua which is said to increase air turbulence just as it enters the fan, which is supposed to decrease noise and increase the fan's suction capacity. Now, without a lab and incredibly expensive testing equipment, it's hard for me to say if these design elements actually work in the real world, but let's assume the folks at Noctua have put them there for good reason.
The fan I've been talking about is actually Noctua's NF-A9x14 PWM fan, and it's sold on it's own too. Specs of this fan include a maximum speed of 2500 RPM (1800 RPM with the L.N.A. adapter), maximum airflow of about 33.84 CFM (24.01 CFM with the L.N.A. adapter), and a maximum noise level of 23.6 dBA (a nearly inaudible 14.8 dBA with the L.N.A.). The NH-A9x14 PWM is a PWM controlled fan, so fan speeds (and thus noise levels and airflow levels) will vary greatly depending on whether you use it in PWM mode or power it from a regular 3-pin fan header instead. Even in PWM mode results will vary depending on what motherboard you have and how PWM is configured on your board.
The NF-A9x14 PWM uses Noctua's long life SSO2 bearing design which has a rated MTBF of >150,000Hrs. Noctua even backs this with a strong six year warranty on the cooler and fan.