For those having trouble getting good quality, I have found that if you use the right method to combine the results of Lukas’ denoiser and the Intel Denoiser, you can get excellent results.
Example .blend
DenoiseCombine.blend (854.0 KB)
Essentially, the detail weights for the built-in denoiser are at low values on purpose, then the node setup finds the unprocessed pixels and uses the Intel denoiser to get rid of the remaining noise in those areas (ie. using the Intel denoiser as a finisher rather than the primary workhorse).
The only weakness is that it could leave just a tiny amount of fireflies, but 99.99 percent of the noise you would have otherwise is gone.