Although I've not owned my 2.5TD Omega (BMW Engine) for too long (few months) I've known the car for a couple of years or so (as I bought it from a friend).
The car has been regularly serviced and oil changes have been adhered to (as far as I'm aware the oil changes are important but they're quick and easy to carry out - other 'servicing' can be carried out less frequently). The only trouble I've really had is when the A/C compressor broke, smashing up the radiator and fan. The car I've got is a '98 and to be honest I felt it was just bad luck that it broke.
I've also currently got an air-bag light warning, but that was my own fault for cleaning too vigorously under the seats and disturbing the wiring.
The car has (and yes, I'm touching wood here) never missed a beat. Started first time, every time. Always.
Economy is fair, but not record breaking by any means. Motorway's are where I do my miles and I'll sit at 40mpg, or a smidge more if I'm careful. Around town it will drop to 30mpg.
Engine power is, again, nothing to write home about but will happily pull you along at motorway speeds without any real effort. I've got an auto-box and wouldn't have a manual (although like I say - I sit on motorways, either crawling in queues or just trundling along at 70, or so).
It's a big, comfortable car though - and I wouldn't swap it for anything else around the same cost / size.