I had it done in both my Jeep and my Passat (When I had it.) The difference on both was errr "Interesting" ..
First the Jeep - now its a 2.7CRD driving two axles.. so fuel was kinda on the heavy side.. at around 30 - 32mpg, it has 130k miles on her so I bit the bullet, new auto fluids & filters, Terraclean on the engine, filters & Castrol The Edge and.. pickup is better more responsive and.. 35 -37mpg on a run.. didn't have that before.. so yes, it works better and the oil stays cleaner longer..
Onto the Passat, FSH dealer servicing, 168k miles, feeling a little lack lustre... Terracleaned and many commented that it felt more like a petrol than a diesel, and it could lift its skirts and go, better than before..
My conclusion is that it does work, emissions are lowered, performance is better, engine temp is cooler and over all its good for the car.. so I'd say yes, I'd go for it...