Although this may sound harsh but in reality the truth is that each persons understanding and perception of the word
silence and the subsequent sensitivity towards audibility varies between young and old, right, left, center so when you ask of how quiet the fan is it may seem silent to someone while another can come in to nitpick on how annoying the motor ticking is at low rpm's.
That is why fans come in with a decibel rating to help users guage how quiet or noisy a fan may be during operation.
IMHO if you're drawn to them due to aesthetics then I'd say go for it if it meets your builds color scheme though you could look into a more neutral color scheme courtesy of Noctua's Industrial and Redux series. You may also be interested in Swiftech's Helix fans.
A couple f things I'd like to add:
Acoustical Noise (12 V): 14,2 dB(A)
Acoustical Noise (7 V): 8,6 dB(A)
^ from 1300 RPM model specs.
Reduction in voltage = reduction in RPM thus the reduction in noise but it also cripples airflow. So what will you intend to use the fan in tandem with? Quieter operation eventually means living with compromises. How big of a compromise you're willing to make is entirely up to you.