Source? Anecdotally many people do. It's actually quite good imo considering the native GUI and features like hide-my-email.
I've been using iCloud since they announced support for custom domains. You can have up to 5 custom domains, catch all and 3 aliases for each domain as low as $0.99/mo
First time I hear about them having custom domains, interesting.
I do, and I am happy with it. I don’t use the Web interface very often but it does the job just fine when I need it. It certainly does not have many features Gmail has, though. Some bits of iCloud are wonky but email works just fine.