Position preference will not affect the way your ads are ranked. The usual AdWords ranking and relevance rules apply. Setting a preference for the first position does not mean your ad will be ranked there. Position preference simply means AdWords will try to show your ad whenever it is ranked in your preferred position, and avoid showing it when it is not.
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Having tried this "new feature" for the past week I can only say it doesn't yet do what it "says on the box".
I did some tests for position 2-10+ for some of my keywords ... it seemed to work sometimes but 6 days after setting things up some of my ads were STILL at position 8!
I've now changed back to the old system, with the same bid / keyword that Google's position preference had landed me at 8 ... and I'm now at position 1 as expected.
I've since spoken to Google support and they cant understand why it happened that way - but are feeding back my comments to the development team.
So it looks like it needs quite a bit more tweaking before its in a fully useable state.
It should be quite a useful tool when its working correctly but at present its too unpredictable to be of much use to me.
So if you try it out and find more problems - feed them back to the google ads team.