Harness connects firmly to the switch, however there are two issues preventing 5 stars. First there is a lock mechanism that doesn't actually have anything to lock into. Second the harness can be flipped and still connect to the switch, so pay attention to your polarity and up versus down. I would recommend putting a mark on the matching sides of the switch and harness to ensure you have it aligned correctly (especially useful if mounting while up into a car's dash where lighting and maneuverability may be lacking).
Switch seems to be decent quality. While there are some markings on the back of the switch, they are hard to see and a wiring diagram/schematic would be very helpful.