Prof. Michael Klein writes:
I made a test series with an existing LEGO scene in Blender 2.81 to compare the Cycles Denoiser with the Intel Open Image Denoiser. I started with 1 sample and doubled it 9 times up to 512.
Please feel free to download the picture wth 5 x 10 HD pictures with a resolution of 9.960 x 11.160. The preview picture in this posting is downscaled. I saved it as JPG in Photoshop with maximum quality. The file is 48 MB.
Here's a short summary:
The Cycles Denoiser looks better when it comes to higher sample rates. It keeps important subtle texture details (eg. bump maps) but even at 512 samples there are still quality issues in the glass parts.
The Open Image Denoiser is not able to keep those texture details but looks better at lower sample rates. At higher samples rates the picture seems still blurred at the cost of missing texture details.
Interesting is that the Denoise Albedo layer will add artifacts/noise to some parts of the picture and makes it looking worse. There's not much a difference between a denoising without and with the Denoise Normal pass in a still picture. Maybe this will be different for animations? Also the head of the Minifigure looks not as good as the Cycles Denoiser result.