I've had my 2005 ST TDCi since it was 5 months old, although I sold it my friend who ran it for a couple of years while I had an Octavia vRS and then decided to buy it back from him as I preferred it to the Octavia.
Bad Points:
Injectors can go, but its not as common a problem as many would have you believe. The initial calibration on the 2.2 TDCi caused excessive wear on the injector tips and when Ford released a new calibration and cars were upgraded (which many were because the software update was also required to fix a software problem with the ESP system which affected lots of cars) it caused them to run like an old tractor. Lots of cars had new injectors under warranty - including mine at 35k, it's now on 80K and it's been fine ever since.
The signs of injector wear are a big puff of white or blue smoke on a cold start and rough running for the first 5-10 seconds. A small puff of smoke is normal, but if you can see it in the mirrors it's a sign of a problem. Worn injectors will not cause the car to break down though - you'll just get increasingly rough running from cold. A full set of replacement injectors will set you back £500-£600 from a Delphi Diesel Specialist, and I wouldn't bother taking the car anywhere else if you suspect problems with the injectors or fuelling system. Common rail injectors can fail spontaneously - they have tiny electrically operated valves in them. You can change a single injector. If an injector fails the car will go into limp home mode.
Clutch and DMF - The clutch doesn't have any specific issues, but the Dual Mass Flywheel can fail. This will manifest itself as a metallic rattle at idle, particularly on start and switch off. Starting problems can be a sign of DMF problems as they drop metal filings into the starter motor. They don't usually fail catastrophically, but this is an expensive job - £600 or so for the full clutch and DMF kit and it's a 6-8 hour job to change as you have to drop the front subframe. Labouring the engine and lots of town running kills DMF's.
The aux belt pulleys and tensioners originally fitted to cars built before late 2006 can cause a resonance in the belt that makes a horrible flapping noise. This will disappear at the very slightest touch of the throttle. A modification kit is available, many will have had this under warranty, but its about £125 for the kit, plus fitting.
The points about the ST rear bumper are true. If the fixings are OK, the best fix is to gently jack up the corners of the bumper until it's level and square, then drill 6 holes and fit some self tapping screws with big heads and repair washers underneath behind the centre panel on the bumper. There is a bolt-on crossmember behind there so you're not drilling into the body or anything. I had a new bumper under warranty, but after a year that started to drop as well. I screwed mine on ages ago and it's been fine.
The diamond cut wheels corrode. Get them painted or powder coated if you want the finish to last.
They are a bit noisy from outside when it's very cold. This engine is 2 generations old now so it won't be as quiet as the latest engines, but it's no worse than a VAG PD engine when cold and it's noticeably smoother and quieter when warm.
The factory fit exhaust is no longer available, and the replacement Ford backbox is about £130 and doesn't have a chrome tailpipe so looks totally naff. I got a Powerflow dealer to build and fit a custom SS cat back system for £250. Check the weld between the backbox and the pipe as they always fail here, but they rot from the inside out so it can be difficult to see.
The back seats can be really noisy where the vinyl covering on the sides touches the plastic trim. Cover with self adhesive felt for a couple of quid off ebay to fix this.
With primarily motorway use you will see 50MPG, mixed use mid-40's, dropping to 40 if you have significant town use.
Decent 225/40/18 tyres can be expensive, but otherwise they are cheap cars to run. They don't need fancy oil and there's no cambelts to change. Other than some of the front suspension parts (which are robust), everything else is standard Mondeo stuff so it cheap and readily available.
On 25k a year you'll save about £1100 a year in fuel (35mpg for a 2.0 petrol vs 47mpg for the diesel).
I've been really happy with mine - the handling is great and whilst the V6 would be preferable when pushing on, it uses 50% more fuel and to be honest most of my time is on the motorway with the cruise control on.