used to do it often in the days before fuel injection. Between the two it seems to work fine, tho the stuff in a can seems to go deeper quicker. I hate what the can does to the neighborhood, however. Water is much more environmentally safe, and considering how much thick heavy smoke a can is able to produce in a poorly maintained engine as it gets soaked up, then b-u-r-n-s out for a mile or two, it's an issue.
I was worried about legal liability this one time--- Iidled the (jeep), drizzling it in, and slowly accelerated the drizzle until the engine quit. waited 45 and gave it a good soaking. when I restarted the car it was awful -- couldn't see 1 foot in front of me, and this had to be awful for the neighbors. so I jumped in and headed out onto the main road and literally left a thick, impossible-to-see-through cloud a mile down the road, and quickly got off. I feared if someone had an accident, it'd be on me. Shut it off. Later started it back up where I was (big parking lot) and just let it idle, then drove home. I was very embarrassed.
I'd never seen one do anything that bad before.