generally it is noise/performance.
however it is not true to say there are no good competition for it.
Cooljag 2400 at 70% basically gives AP-15's performance and with that room to do better.
Reeven 2000 at 100% is also pretty darn good with static pressure / noise.
AP-15 however has been around longer, and for those that 'water cool', we tend to have enough radiator space to look for the 'quietest' fan, and the AP-15 just happens to be around during the time when we are all testing.
as a Chassis fan though, it is not nearly as good.
it is really a radiator fan only.