Right, well let's see. There are lots of different versions of these.
Pre 1997 they are plain old TDI with a basic computerised injection system - these are reliable, as long as you change the cam belt!
1997 til around 2004 ish depending on which VAG brand and car, they are the older PD engines. These are the ones that deliver a massive slug of torque from 1200-3500 rpm and then nothing. To make progress as fast as possible you shift well before the red line. Some find this annoying, but having all the torque concentrated means that in that band (ie where you do all normal driving) all you need to do is ease down slightly on the pedal and regardless of gear or road speed you just get flung forwards - good fun 
My Ibiza 1.9 was like this, 53 plate.
Then they added electronic injector control to the PD engine, which is where the 2.0 TDI comes in. This was between 2005 and 2008 on Passats I think. They have a much flatter torque curve, my Passat pulls well up to the vicinity of the red line, building all the time. Not as much of a bonkers slug of torque low down though. They sound less clacky but fairly weedy like a coin in a matchbox. Commonly comes in 140bhp which is a nice useful amount of power without being silly imo. I think Seat changed their cars over to this engine later, based on the test drives I did. Some batches of these from certain factories had catastrophic oil pump failures after 40k miles or so leading to engine death. This was recalled, and any surviving engines do not have this issue. You can find exactly which engine numbers are affected online. There is also some suggestion that the injectors are not as reliable - but some say this is the just wiring not the injectors - despite some garages changing all the injectors anyway. My car had some slight stuttering on very light throttle, cold engine, low speeds when I got it - now after lots of long motorway trips and some ragging on the autobahn it's amazingly buttery smooth. Diesels really love being ragged.
After 2008 they have common rail engines. I have only driven one in a van but it was buttery smooth. No clacky clacky at all, just a lovely hum. However the van was so under powered I can't make any claims about power distribution.. there really was none to distribute
Oh for info - my Passat is a 2006 2.0 TDI saloon with the DSG auto, and I get 52-54mpg on long trips including 80mph motorway driving, and even as much as 63mpg occasionally on the trip to work which is part motorway and part 60mph open road. Not bad for a big auto
How the hell do they pass the Euro 4/5 test?
