for those on linux, here's a simple .mailcap that i use:

    text/html; w3m -M -T text/html %s
    image/*; feh -F %s
    application/pdf; okular %s
    application/msword; oowriter %s

(i also use procmail, with procmail-lib, to filter into monthly mailboxes, and the wonderful mairix http://www.rpcurnow.force9.co.uk/mairix/ to provide fast search. in fact, for me, mairix is the most important part of all this if you want useful, fast email - but i haven't used notmuch so can't compare)