I just sold a 1994 Audi with more miles than the space shuttle and I'm confident in saying that I expect it had less rust than your 3 year old Omega does. Put quite simply, Vauxhalls (and GM in general) are utter junk in terms of build quality. Let them know your opinion on the matter, and make sure you tell everyone you know of that same opinion. It won't help your case any, but the more people who are warned away from this manufacturer the better!
The Omega suits me great for what I bought it - a cheap, comfortable and spacious car for doing all my mundane motorway driving and lugging stuff around. It's also RWD and has quite a good chassis for its size. However I'm under no illusions of the standard of build - I've been driving since I was 17 and never had breakdown cover as I didn't think I'd need it, but I do now! A few months after buying it my dad was using it and got stranded when the crank sensor failed (very common fault); maybe I'm being unfair as it's been fine ever since but I can't say it fills me with confidence. I'm prepared to put up with things like that and the rust though as they're cheap to buy used for that very reason - few people want them due partly to image but also lack of quality, which drives down the used price.
If you want a car that won't rust, buy an Audi (or indeed just about anything except a Vauxhall!). Unfortunately rusting is just part and parcel of owning these cars, and VX are obviously well aware of this hence why they're very reluctant to budge on the warranty issue as they'd go bust trying to fix rust on just about every car they've shipped in the past 10 years :O
Vauxhall, in my experience, are no worse than Ford/VAG (Audi aside) or most others for rust. Prefacelifts did have an issue with just under the rubber strip by windows, granted.
Vauxhall are let down by their dealer network, which is almost as bad as Renault and BMW, which is a shame, as the cars ain't that bad (Astra-H excluded :p).
Every car I've ever owned (or been responsible for in case of company vehicles) has let me down - the Omega has only done so once (crank sensor, like yours), so again from my experience, no worse than any other car I've owned/responsible for, in fact better than most. Even my supposedly utterly reliable Honda broke down with a known, common electronic failure.
I bought my MV6 for a specific reason, and only intended to keep it 6 months. Thats was over 3 yrs ago. There is nothing to replace it with for what I'm prepared to spend (5 - 6k).
I'm certainly not a Vauxhall fan, never have been, but the Omega ain't that bad really. Obviously, it won't suit everyone. Generally, most problems are due to lack of proper maintenence, though it has got one or 2 bits that probably could have been better designed (eg, running of crank sensor wiring).