Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) l saw her taking her trousers down, and l definitely glimpsed some cellulite down there. Good decision, yeah. All actresses are as mad as snakes. - Tones, what do you reckon ? - Never met her, never want to. - Brilliant. Max ? - Absolutely. Never trust a vegetarian. Great. Thanks. Brilliant. (..)1999 Notting Hill