also mercs are prohibitively expensive, most people use slaves which are useless unless you have a bunch.
But absolutely worth it if: ( this goes for mercs only )
1. You know how to use them.
2. You have a leader and/or slaver build.
3. Have at least one. ( lol )
4. Only purchase the melee ones and like.. give them shotguns.
5. Don't foolishly get them killed by making it to where they are standing in an entrance in a gaurded town. ( Like i have done several times..

)