I hear a lot of people swearing by the stuff and some others calling it snake oil. I have come across a few instances of claims that Seafoam fouled up an O2 sensor. I haven't seen any real tests on the issue so I can't say if it's something that actually works or is just all in your head. I had planned on brushing some on the valves of a spare head I had and see what it does, but ultimately gave away the head before I could do it.
I did give my Miata the "suck into the vacuum line" Seafoam treatment about a year and a half ago and it didn't cause any noticable changes one way or another. I don't know if that's because it didn't work or the engine just didn't need it. I'll likely try it on my 944 soon since it seems that the chip I had in it until recently increased the carbon build-up.
Now, if only there was an easy way to get a "before" and "after" view of the cylinders...
BB.