Was told by the garage I had it done at that it works better on the diesels. I use mine a lot around the city / town and carbon really builds up. I watched it for 5 minutes and the crap that come out was mad
Terraclean offer two specific packages. One which is priced around £108 for cleaning the fuel system and part of the engine. The other package can include cleaning the inlet manifold, EGR, DPF and a supposed more 'thorough clean'. Was planning on having this done on my Audi but instead has to pay for a new inlet manifold as mine cracked.
Terraclean advise for their 'deep clean' to be done every 15k which could be quite expensive if you do get it done every year. I know a few people who have had it done on older aged cars to reduce the carbon build up and clean the engine and they seem to be happy with the results. As i said, i haven't had this done myself but it seems to be a posher version of a fuel additive.
If you go to their website and look at their images, they show vehicles EGRs before and after and yes, it does certainly look like it does a good job! The way they do this is by over riding the fuel system in the car, putting two additive canisters in their machine, hook the machine up to your car and basically run the cars engine for around 20 or so minutes revving it here and there.
Personal recommendation would be, if you want a thorough clean, do it - looks good. but all depends on how your car is being driven etc