Well I bought the 'trim cutters' as they seemed a good idea, and they are for internal corners with plastic trim.
However for external corners I always use a mitre block and hacksaw with fine teeth, and for all with metal trims.
You just cant do external mitres with a trim cutter as you have to stand the trim up so to speak and this just doesnt work in the cutters.
For finishing, either fine sandpaper or the rub stone I find very good