First year, the A/C went odd.
Second year, front brakes noise and squeaks + continuous front tire wear + steering mis-alignment.
Third year, total brake system failure + engine overheating + minor oil leaks + rear curtain hang-up + front electric passenger chair fault.
Finally, in May 2003 (one month after the warranty expired) I had a major engine oil leakage. The engine was completely overhauled and dismantled (not free of course).
The car is generally good, but the engine noise should be reduced.
Cheap wood trims are used in the cabinet.
Alloy wheel rims can be easily damaged by road condition (17" sport).
More hp should be added to the engine.
The rear trunk cannot be opened while the key is in ignition position!!
I'm also from Qatar and as you I own a 1999 Omega Executive Sedan 3.0 V6 24V Automatic (I Don't know which model, you must has, if you could tell me if it is a Sedan or the Caravan, model I will appreciate very much).
I Bought it in 1999 (new) and until now I didn't have any kind of problem with the car, neither a flat tire, in 52.000 Kilometers on the clock the only running cost which I Had with the car, it was Petrol and Oil changes.
The dealer service was satisfactory in all times I had visit for periodic reviews.
For what can I see in your review, you must got unlucky in buying a model, which the previous owner doesn't take good of the car or You got a model with many problems from the manufacturer (I'm saying this because, you bough the car in 2000 with only one year of owning) and with all respect: I Think you didn't take care good of your car.
About that thing you say the Omega is an Average German Sedan, well it's a a question of taste you know, in my opinion the car is Great!
All right Opel's doesn't has the same prestige of BMW or Mercedes-Benz, but remember: Audi are luxurious Volkswagens, the Audi A3 for example it's a Golf with the Audi badge, that's all.
If you compare the Omega with a BMW 5-Series for example you will see the Omega has a great value for money, because costs almost the half of the BMW and has the equipment. By the way previously the Omega, during 1995-1999 I have owned a 525I Sedan (old model) and I tell what: The Omega is not so prestigious as the BMW, but is so good as the 5-series.