I visited my nearest TerraClean franchise yesterday and thought I'd post my experience
First impressions count - I looked at the facility and wondered whether I should bother, but having got over myself, I decided to proceed, the mechanic was friendly and knowledgeable, and as I explain below, turned out to be a sound bloke.
I didn't take readings before and after so can't verify results, the only thing I can offer is my perception.
For those unfamiliar with the process, and for Diesel engines, the engine is isolated from the fuel tank and runs from 50/50 mixture of diesel and TerraClean product stored within the TerraClean machine itself hence the red and black fuel lines in various photos posted by BuckMR2
After hooking up the TerraClean machine to my 2.2D with 36k on the clock and running the engine for ~15 minutes an alarm sounded on the TerraClean machine, it turned out the plumbing was not quite right and the return line was in fact filling up my fuel tank and had emptied the machine. Fortunately for the mechanic, whilst an expensive mistake for him, had lost ~ 2 gallons only. With plumbing sorted, the engine ran for a further 20 minutes or so - I didn't take exact timings. The engine was left to rest for ~5 minutes then run again for a further 5 minutes. Given that I had a full tank the effective dilution ratio was about 12:1 so I expect to get an enhanced clean for the remainder of the tank

Turned out nice again.
We all know diesel engines are noisy but I perceive the engine noise reduced during the TerraClean process - entirely subjective observation - but the engine was hot when the machine was hooked up as I had just driven ~35 miles to get to the garage, though there was ~15 minute wait whilst the pipe connectors were souced to hook the machine up
The second thing was a custom remap, I was going to have a tuning box fitted but the socket from the tuning box would not accept the plug from the common rail, the key slot was offset by a couple of mm and rather than file the key off (as that would have been a dead-giveaway to the dealer - car still under warranty) I decided on a custom remap instead. The download / upload time surprised me - the download took ~15 minutes IIRC then it was sent off for tweaking before being uploaded. I went for a 50/50 economy / power remap
Due to the fuel issue mentioned earlier we were running late and another customer turned up with a Petrol Turbo 4x4 with 160,000 on the clock. The mechanic hooked up the petrol TerraClean machine which has two cans black / red sticking out of the top and proceeded to run the process. From the smoke coming out of the back it was clear there was a lot of carbon being burnt off. Unfortunately whilst looking around the engine, small bubbles could be seen coming from the head gasket in one corner and due to the oily residue down the side of the block, it was clear the head gasket had been leaking for a while. Needless to say, with the engine running and TerraClean machine hooked up, the engine overheated and it was obvious the head gasket had gone. There are a couple of factors to consider - it is unlikely the TerrClean machine was to blame as the oily residule indicates a pre-existing condition. The owner used the car for short journeys where overheating would not necessarily have been apparent. But it could also be said that the TerraClean process exacerbated the problem by cleaning up the injectors etc therefore improving power which might have been the straw which broke the camels back. The mechanic was well within his rights to ask for payment but decided not to charge - he even offered to sort the head gasket out at an amazingly cheap price
All in all an interesting afternoon
In conclusion I can't say the TerraClean process did anything noticeable to improve my engine - and the remap may have done everything - however, the engine idles at a much lower rev count, fuel economy has gone from 38 to 44 mpg and the power band is extended and has much smoother delivery. I am delighted with the results though and will certainly use TerraClean again, I am considering the same approach for my wife's diesel Hyundai