I ran one for a while, not sure if it benefited or not. Yes it collected muck, much of it condensation mixed with oil mist so may have looked worse than it actually was. I think the jury is out on this TBH.
I removed mine and conceded that direct injection will always bring issues with it in our case since there's no fuel/air mix over the inlet valves.
To help keep the engine at it's best drive the car regularly. Get EGT's and cylinder temps good and hot, use high quality oil (look up Ester based), use good quality fuel (Momentum or V-Power, whichever takes your fancy) and avoid variable servicing along with journeys which are so short the oil never comes up to temp.
Other than that, these cars seem to be able run fine with a moderate build up. Schedule a manual clean every 4 years if you own it long enough. If you include £5 or £6 a month for this as part of your maintenance schedule it's not too onerous.
JKM told me that they've seen some cars that are particularly bad and others that are surprisingly good for similar or higher mileages which to me suggests that they could be affected by a variable, which is the driver possibly.
