I've tried Notmuch after considering it and mu4e (they looked very similar, don't recall anymore why picked one over the other), but it's not my cup of tea. Also none of these eliminate mpop, msmtp or procmail, I want to use Elisp tools instead of these (the actual one that has no equivalent in Elisp is procmail). I currently use mairix, which is similar to notmuch and mu in that it provides fast search across my mailboxes (which are my inbox, and quasi-periodical archive files [archive000, archive001, ...]), but it supports mbox format which I like for various reasons (doesn't eat up inodes, easier to check integrity [just hash the single file], no file-name portability issues [which I often had with Maildir], simplicity, out-of-the-box support on any Unix-like systems, etc.). With it I can treat multiple mboxes like a single one, and I've written an org-mode link type for mairix so that I can easily link from my org files to an email (via message-id) or a mairix search w/o thinking about which archive file the mail was in.
Edit: I also don't really use tags etc. to classify my mail, and don't have a plethora of email as I delete most of it after reading, so a list of all I have is the best interface for me, which Rmail provides.