I know folks who have been involved in this nonsense since its inception as the "Manhattan Project."
All I have ever heard is that:
a) they place a lot of magazines/tracts
b) other religious nutjobs come up and try to convert them (this is in NYC, BTW)
c) people who are "evidently" apostates occasionally approach and bring up "controversial" topics or call them a cult. They just try to ignore it.
What I have never heard is:
a) we started a bunch of bible studies doing this
b) so-and-so came into the "truth" by table/cart witnessing
My thoughts: this has nothing to do with increasing membership by reaching more people. I think it has more to do with the mind control aspect of this thing.
In regular FS, you aren't always "on": you ride out to the territory with friends, chat, go for coffee, etc. Only when you knock on the door are you in full JW mode.
With cart witnessing, you still get to chat with your buddies, but 100% of the time you are standing there you are in full JW mode - you don't get a break by walking down the block a bit or getting in your car. You are ON 100% of the time. I think that has a HUGE influence on how people view themselves. In other words, It's all part of the brainwashing crap.