Definitely depends on the mood.
To clear my head when there is stuff going on and do some thinking its definitely alone, have had many of these cruises. Most memorable was last year when I just got my license I cruised from here (brisbane) down to lismore and back one school night. Most of these cruises are on deserted roads at after midnight through where there are barely any cars or houses around, and with the way I drive then, I'm definitely not allowing any passengers.
Around town just out for a drive definitely get mates to come with, 2-3 people in the car is about it though, otherwise it's just to crowded. Best cruises with mates are the ones with around 5-10 cars with 2-3 people per car. good mix.