I got the 2.2 auto and get (averages) 22mpg cold/around town, 27mpg mixed around town, 32mpg on a town/rural run, but 35+ using cruise on a run. Managed 41mpg on a warm run the other day, flat A-road, 55mph.
For the record, my overall average is 31.9mpg-includes stops/start driving and A-roads.
I love the 2.2 16v and think the economy fair for such a heavy car and would disagree that it is underpowered-mine flies when baited and seems a good compromise between power and economy.
A manual would be cheaper to fuel, but auto and cruise suits the car-it is a cruiser.
Economy is down to technique-plan ahead, be smooth, avoid unnecessary stops and starts-it is a heavy car to accelerate!
Before being too hung up on the economy, look at your average mileage per year and work out the actual £/pence fuel cost difference between 25 and 35mpg. I bought my Omega knowing it would be thirsty-but comfy, safe and big enough for five and luggage.
10mpg difference over say 10,000 miles pa only costs a small amount in the whole scheme of motoring and look at the huge premium to buy a diesel in the first place-if you do sensible miles, the petrol does makes sense.
Note the April 2010 tax increase on cars registered after 1st March 2001-goes up to £245pa for any petrol Omega due to high CO2, instead of the blanket new £205pa on the old system for any car over 1549cc. Knowing the government, the CO2 based tax will just rise and rise and the pre-2001 should stay the same-perhaps the gap will widen? I chose a pre-2001 car deliberately.
See:
http://www.direct.gov.uk/en/Motoring/OwningAVehicle/HowToTaxYourVehicle/DG_172916So-2.2 16v; 25-35mpg daily if well maintained and driven carefully, quick enough and cheaper to buy than the desirable diesels.
Depends on what the deal is?! If you find a cheap 2.2 diesel (!), great-but the ££££ difference is a lot of fuel...