I had never bothered with iCloud Mail before, but I started using it recently as a secondary mail account when they introduced custom domains.
It’s been fine so far, the service seems reliable, no spam problems, and it was easy to set up initially. I find the Apple Mail client good enough for my email needs so no issues with that, I can’t speak for how iCloud Mail works with other clients however.
It’s good enough that I may eventually switch to it as my primary email provider.