I use 3x 4mm Castle connectors with 12 AWG wire to connect my ESC and motor. Out of curiosity I measured the resitance of this type of connection vs a length of 12 AWG wire on its own tonight.
My multimeter was set to the 200 Ohm setting and the resistance of my leads was 0.7 Ohm. The bare wire was 0.8 Ohm resistance, 0.1 Ohm minus the resistance of the test leads. I did a worst case solder connection on the Castle leads and my resistance varied between 0.8 and 0.9 Ohm corrected to 0.1 to 0.2 Ohm if the test leads were factored out.
Basically the Castle connectors add 0.1 Ohm worst case over a 12 AWG wire connection of the same length.
The Castle connectors are a different design to the typical 4mm spring loaded connector which is what helps them achieve low resistance I think. For me they add a level of convenience when I swap motors/ESCs between vehicles and I'm really not going to see 0.1 Ohm extra resistance on the track. They haven't separated in a crash yet either. I've heat shrinked the bare connectors so they don't short out if the wires touch.