I may be old and half blind, but I'm not seeing a difference.
Tell that to the others in here. I just posted clear evidence backing up what I said, and they still argue against it. Whatever. My advice is aimed at those of us who do not have deep wallets and are stuck with a low end card and FHD 1080P rendering. I just posted the proof and they ignore it.
What else can I say? I posted my own honest tests with an HDRI background (as one of them suggested, my tests uses an HDRI background) and the proof is in them.
I'm not saying everyone should use it, if you have the hardware and t he time to wait, by all means, avoid denoising, you probably get fast renders anyhow, but for the rest of us, denoising is a fast alternative which produces some really nice results, really fast and for me, as seen in my example I posted, I see NO DIFFERENCE. Period.
Also notice I wasn't attacking anyone in MY POSTS, I originally posted in order to help someone who was having slow render times and MY post was attacked by the anti-denoising zealots in here. I didn't attack anyone, I tried to help, but I may not bother to help anyone again in here if this is what I get for my efforts.