I've used it in both my trucks; through the brake booster vacuum line and in the gas, never in the oil.
I like the vacuum line because it cleans out the intake manifold. I did it with a hot engine so it would stick better. You need two people. Pour in at least 1/2 the can, shut the engine off, let it sit for 10-15min then start it back up. Some say not to over-rev the engine. I just keep it below 2,000 on initial startup. Let it run around 1000-1500 for a little bit, then go drive it like you stole it. I like doing a WOT run onto the interstate so you can't get a ticket easily. Opening it up basically just allows the maximum amount of air to get sucked by all the gunk to break it loose (at least that's my understanding).
The first time I used it in the S10 it ran really rough on cold-startup after using it. It really chugged and bucked. I think it broke some gunk loose that it didn't like. After a few days, the truck was running just fine.
I have not used it in anything carb-ed (my motorcycle, lawn mowers, snow blower) so I have no experience with that. I know I read one guy said he used it and later had to get the carb redone on his mower. I just don't think its worth while on a small engine like that.