This is a chronological account of my experience with the hesitation or juddering at 1500 rpm, failed SCVs, field remedy 1957 and so on. It resembles that of m8internet (I think) some time ago.

Bear in mind I'm living in Spain, so this car is an Opel, the prices are stated in euros and the mileage... ok, I can convert km to miles

I bought an used 3.0 V6 cdti Signum auto from a renting company. The car was first registered in May 2006, so it's a facelifted model and the engine is a Z30DT (but without the new SCVs fitted from September 2006 in this engine).
The car was sold with a one year mechanical warranty, it has always been serviced by Opel dealers, it had 40000 miles on the clock.

I soon noticed the infamous juddering, vibration, hesitation, shaking (you name it) from near 1500 rpm to 2000 rpm, which was felt clearly while driving in 5th and 6th gear, sometimes also in 4th.
Not being speed-dependent, I discarded a problem with wheels, transmission shafts an so on.

I did a little research on the web found this site, found the info about the SCVs, and thought: "it must be this: good I have a mechanical warranty".

So when the occasion presented, I went to the local Opel dealer, told them about the problem, and told them to talk with the renting company about the detail$.
I received a call from the renting company saying that the Opel dealer pretended to change 6 injectors, EGR, exhaust gas cooling system, and some throttle thingy for 7000 euro, and if I didn't mind them asking for a second opinion... so the car was sent to a diesel injection specialist, who didn't found stored errors and could not find the hesitation whilst driving (as it was an intermittent problem). They added a fuel injection cleaner, anyway, returned the car to me, and it seemed that the problem had disappeared.

But it didn't. It returned a few weeks later. I waited until the next maintenance service was scheduled: I went to the dealer again, this time telling them about "field remedy 1957". They downloaded the software mentioned in the remedy, as it seems to give more details about some of the errors related to the failing SCVs.

We are approaching the end, don't worry. A few weeks ago, the EML light went on, the car entered into emergency mode (the engine runs over all the rpm range, but there is not much acceleration left, so you can go 100+ mph downwards, and not being able to overtake a bus at 60 mph in the steep inclines of Somosierra, in Madrid). The EML and the emergency mode went off after "rebooting" the car, but it reappeared a while after.
The error waas P0093-52, in case you are interested: big fuel leak, but I didn't smell the fuel, so this time I got a DTC to hang on and get the SCVs replaced.

Again to the dealer I went, and new SCVs I have, paid by the renting company from which I bought the car: just in time, one more month and I had be paying the bill.

Speaking of the bill: 714,73 euro (+18% VAT), of which 194 (+VAT) are for the SCV kit (ref 8 19 348), 6,6 hr to replace the fuel pump, some more hours charged for the diagnosis and reprogramming and whatnot, and some refrigerant that needed to be refilled (as it is required to disconnect refrigerant hoses to reach the pump).

So that's it, the car has now 63000 miles and the hesitation has gone.