"We also ran brief test results comparing the cooling efficiency and noise of the Scythe SFLEX SFF21F 64cfm/28 dB-A fan that was used in Thermalright reviews and the SilenX fan used in several of the most recent reviews. Cooling results were similar with either fan on the Noctua heatsink, with the SilenX providing better cooling on some overclocks. Both fans were quiet and came in below the noise floor of our test system, but the SilenX was subjectively quieter than the SFLEX.
Note which of the Scythe fans they tested with: The SFF21F rated at 28 dB. Scythe also has the E and D versions of that fan which are rated at 20 and 18 dB respectively. So for the SilenX fan to be subjectively rated only slightly quieter than a fan running at 28 dB isn't saying much, at least not in SPCR terms.
Please note that the SilenX provided better cooling than the SFF21F on some overclocks. This proves that both fans were pushing about the same CFMs on the HS. Also, the noise level of the Scythe is specified conventionally, and the SilenX highly unconventionally - the two numbers cannot reasonably be compared at all.
I also own a SilenX 120 and a Scythe S-Flex 120, in my case the "11dBA" SilenX and the SFF21E. I think my SilenX is quieter for the same airflow, but it's not so much that the SilenX is so quiet as it is that the S-Flex is so noisy. The S-Flex fans are widely reported to be quiet fans, so my one sample must be bad. My SilenX has about the same noise level as my ~6 GW NCBs and one YL D12SL-12. It starts at a lower voltage than the other fans and the fan blades come closer to the case "duct" - two factors that make me favor the SilenX for very low RPM applications.
A fan is a fan is a fan. Putting a high, low, or indifferent noise specification on it does not change the fan in any way. The folks who criticize the SilenX fans seem to do so because they don't like somebody associated with the SilenX company, or they don't like the specification. None of this has anything to do with the fan itself.