First off, the denoiser. As expected, it’s not a magic bullet; if you don’t have enough samples, its like running a Photoshop blur filter over the image, However, give it enough information and it does a much better job. As an example, I’m doing a large cityscape animation (currently rendering in R20 the background) and even with four progressive passes it’s still pretty noisy. I just did a quick test, and If I’d had R21 yesterday, I reckon I could have used two passes plus denoiser and it would have been: 1) better looking, and 2) finished this morning. So denoiser can be like having double the CPU cores. Kinda.
More experimentation to be done – especially using with ProRender – but so far, it’s a tentative thumbs up.
EDIT: Okay, so I may have been a bit hasty. It still adds jitter during animations, but at least I’m losing one progressive pass – and the images do look nicer.