I answer quite a lot of questions on the Exim mailing list. Most of my experience comes from desigining and operating mail rewriting and scanning hubs for medium-sized operations - up to single-thousand domains. I've also participated in the design of ISP-scale mail systems for several UK ISPs.
My favourite Exim topics are ACLs, MySQL integration, Exiscan and ways of integrating external functionality using ${readsocket and so on.
I'm collecting ideas for improvements to the Exim website here