When we had a demo on the system we ran it on a mk5 golf gti for a half hour and it did begin to soften the crap on the top of the valves but it would've taken a good couple of hours to remove it completely.
Apparently the system uses negatively charged particles that are not effected by air direction which allows them to travel backwards up the intake track in order to clean the manifold, egr etc - how true that is I really don't know.
I do rate terraclean I have to say. On the demo I watched the live data, misfire counters in particular and it reduced a pretty prominent, every second or so misfire into a once a minute occurrence.
Both cars felt smoother and sounded quiter as well!