I had MK5 GT 170 for just over 4 years. Had problems with the DPF getting blocked once at just over 3 years into ownership. Apart from that, I had 2 faulty central locking control modules - no other problems.
As far as MPG was concerned; used to see upper 40's on a decent run. Must say, I agree with dubber36; I'd go for a MK6. Really liked my MK5 but like my MK6 so much more.