I have not used Seafoam on the G6 yet. However, I do have a lot of experience using it on my previous car, a 1995 Saturn SW2. As ya'll've probably heard saturns tend to drink/burn a lot of oil. Using Seafoam directly in the intake was great for cleaning out the top end of the engine and the intake manifold, I also added it to the fuel, about one can per full tank (12 gallons of gas) a simple procedure, pour in seafoam, add gas, done.
One time when it was being a real pig and eating a ton of oil I waited until I was about to change the oil and added about 1/2 a can directly to the engine oil. I ran the car for about 70 miles with seafoam in the oil and then changed the oil. The consumption went from about 3 quarts per oil change to 1. So yeah, it works. I can't really speak to it's effects on longevity except to note that when I wrecked the saturn (70 MPH spin to smash into guard rail on Route 68 in West Virginia) the car had almost 160000 miles and ran like a champ.
I used to really like fogging the mosquitoes in the summer time. The procedure was really easy on the saturn, simply suck up seafoam using the pcv valve while goosing the throttle to keep it running. Then watch as the fog comes rolling out the exhaust! It will work with water as well, but the seafoam really does a nice job of degreasing the top of the engine/valves etc...
I think that when I do the oil change on the G6 I may put a can in with a full tank of gas just to clean out the injectors a bit.