Stack up on Acc and Haste if your not at the level for haste gear fill slots that can't hold Acc with Atk and by all means please avoid full timing str rings.
As for what a mnk is good in I'm yet to find something it's been "useless" in.
Dynamis you may not spike as often on a mob like sam's would but your punching does add up.
Sky definately a weaker point but can always go old school with a chi blast set nice to see 1500+ numbers on kirin.
Nyzul works wonders since with the proper gear you can tank things rather well while /nin not to mention a Fanatics with counterstance on a boss floor is always amusing.
In the end Mnk will be as useful for you as you make it, the harder you try to improve it the more useful it becomes I'm sure not everyone will agree but eh thats what that whole opinions are like *** saying is for.