This conversation comes up over and over, plain and simple denoising removes image fidelity. The best thing to say is if you like it and it works for what you want to accomplish then fuck what anyone else says. As long as your happy with the render then go for it.
If you are looking for confirmation then you will get that here. A lot of people use short renders then denoise. Just post the images and don't tell anyone they are denoised and you will still get as many likes.
Denoiser is best used in scenes where there are sharp differences in lighting and part of the scene has either insufficient light or light reflected from other surfaces (such as light walls, ceilings, etc.). In the case of direct rays from the light source denoiser does not give much effect, which, by the way, can be seen in the test renders.
Noise reduction saves rendering time in exchange for its quality in small details. The visibility of such details always depends on the scene itself, the lighting setup and other parameters that are not always obvious. In general, details in shadows are more sensitive to noise reduction and give more artifacts, but Iray is usually powerless to give good averaging there at all, unless you set 20000+ iterations.
