By far the most frustrating thing about plastering jobs is the drying times. This is especially true with inside corners - with a knife you pretty much can’t do both sides the same day because you’ll always end up messing up the opposite one.
I finally gave one of those corner tools a try, and the result is fucking mint. The customer is going to be so happy with these flawless corners tucked behind the curtains in their living room.
I’m a big fan of the corner trowel, but you’ll still need to bust out the putty knife at the ceiling where the three inside corners meet. It takes some practice and finesse, but you can get it nice without needing to wait for any of them to dry. Having a knife that’s deliberately a tiny bit dished so the corners don’t dig in helps (and is super handy in other places too)