Testing:
I gave the fan's two main tests, one was listening to sound levels while installed in the case and the other was comparing them with other silent fans I have while outside of the case so I could listen closely.

SFF21D - The only way I could hear this fan was to hold it right up to my ear. Anything further and it was quieter than a whisper and practically inaudible. There really is nothing to compare this fan to, if you can't hear it, you can't compare it!

SFF21E - This is the best noise:performance ratio fan in my opinion. The 20.1dBA is incredibly quiet, when you get it in the noisy environment of your computer case it becomes very hard to single out the noise of this fan. The only way I could pick it out was to shut it off during it's operation. I had one other fan to compare it to with a similar sound rating, the Noctua NF-S12. In terms of sound, they were both equally as quiet, which is a good thing! In the case, I couldn't notice a different at all. When compared head-to-head outside of the case, I found the Scythe fan to have a slightly more noticeable whine in the motor sound, but when installed in the case this couldn't be heard over the sound of the other components, that's how slight it was.

SFF21F - The SFF21F is a more powerful fan, being designed to move 63.7CFM of air. It was loudest of the three just as the ratings imply. If you need high performance cooling this would be the ideal fan for you because it is still quieter than many competitors in that range.

Conclusion:
Offering 3 different silent fans in the S-Flex line, Scythe is sure to have a fan for everyone with this series. Whether you want good airflow from an inaudible fan (SFF21D), high airflow from a louder (but still very quiet) fan (SFF21F), or a good airflow to low noise ratio from the SFF21E, they have you covered. These are three excellent fans and I wouldn't hesitate to recommend them to anyone looking to build a quieter computer system!

Pros:
Quiet operation
MTBF of 150,000 hours
Included 3-pin to 4-pin molex adapter with RPM sensor
Three different models available to suit your needs best

Cons:
Should include rubber shock mounts for installation too