Noise is one of those very difficult things to quantify in a uniform manner that applies to everyone. Can't really just put a number on it and have everyone draw the same conclusion. The truth is, what some people find as horrible noise another person may consider it excellent. It's all in how sensitive to noise each person is, their expectations, whether they are more sensitive to retaining noise AND detail, or reducing noise and often smearing detail. And different types of photography lend themselves better to using high ISO without as much penalty from noise, due to good overall exposures or lighting in their situations, versus someone shooting in very dark conditions and regularly underexposed, where noise tends to be much more noticeable and destructive to details. I for one have never been as 'sensitive' to noise as some others - I don't mind a little grain in a shot, especially if the details and color are still there...I prefer the noise to a completely noise free photo that shows too much NR. For many years, I've been shooting at ISO6400 even on cameras WAY below the performance of today's systems - even on APS-C rather than full frame. Today's APS-C cameras at ISO6400 are orders-of-magnitude better than those 3-5 years ago, so I am in bliss at the improvements I can get with no post-processing required.