Same engine as fitted to Golfs, Octavias, A3's etc. so there should be loads of people out there with them who can comment on the engine side of things.
Mate of mine used to have the Golf GT TDI thing when it came with the 140bhp 2 litre. I think they later upgraded it to 170bhp. He really liked that car and it seemed a pretty good engine. Only thing with it, was that it drank oil. He used to keep a bottle in the boot and would have to top it up fairly often. He was doing a lot of miles in it though (probably 40k/yr at least).
You should see 50mpg easily out of the 140 diesel.
I've got the smaller Ibiza (based on the Polo/Fabia) with the 1.9 130bhp TDi engine, which is the older, more clattery PD engine. I think the 140 & 170bhp ones are common rail.
From an ownership point of view I love my car. It's been reliable, economical, plenty quick enough & cheap to run. Perhaps not massively helpful given that it's a different model, but the car has definitely exceeded my expectations.
With regards to handling, in my experience Seat set their cars up a bit harder than VW & Skoda to make them feel sporty. They go round corners well, but my Ibiza rides very hard compared to it's Polo/Fabia equivalents and is very crashy over potholes.