Imo they are one of the best 120mm fans and produce a high airflow of 83.63cfm.
Many people say that the airflow values provided by manufacturer aren't to be taken seriously but in Viper case I think Akasa being a respectable company are telling the truth. It can go up to 1900 RPM and slightly more and at full or near full speed, they sound like mini-jet engines but this is also telling you that they are pulling lots of air in. Besides if you look closely at the fan blades, they resemble fan blades of jet engines.
Most of 120mm fans I have seen are mainly in 56-65cfm region.
I think the other ones are scythe series IIRC.
I think the main competitor to Akasa Viper is Scythe Ultra Kaze 120mm 2000 Rpm fan or the Scythe Kaze Jyuni 1900RPM Slip Stream 120mm Fan
http://www.overclockers.co.uk/showproduct.php?prodid=fg-022-sy