Air con is a damn useful feature, winter or summer.
In the winter it is very helpful in defrosting the windows and is a huge help to stop them steaming up when it is very wet (not unknown in Scotland...
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Air con in the Omega is actually very very good.
If you don't have it on normally, I do recommend runing it at least once a week for a few minutes to keep the system lubricated.
Bacteria will certainly build up in the system if it is not used. This can be treated easily enough. You buy cans of stuff to do the job in Halfords.
The best setting is personal preference. If it is climate (digital temp settings) then choose your preferred temp and press auto. It will then automactically keep the cabin at that temp regardless of what is going on outside.
If it is manual, ie rotary dials, then set to a mix of screen and front vents at a temp that is comfortable.
ECO button disables the air con.
Generally I drive with the climate set to 20 degrees and on auto (when I get to use the MV6!)
If you set it to cold, and it is not freezing the **** off you, then an air con service is highly recommended!
Expect to pay £60 ish for a decent independant.
I would not buy another car without air con, just too good a feature.