Custard has some good points. I guess everyone should read that especially newbies. secondly, most of you seem to be forgotted than QS is also dependant on LP loading speed. How fast ur LP loads is a factor in QS. Bejwelled, you are right. There was a major update to QS about 15-20 days back. QS is now real time unlike static, that means each time your ad is triggered a QS is calculated. So what you see on your ad group is a general QS not the actual QS. This means you will never know ur QS. Overall lot of people have been downgraded with their QS. I was at QS 8-9 on all my ad groups/keywords now lowered to 6/7 but i am paying less CPC for even higher position so it seems what we see in our ad group is really not what it is out there.
but I downgraded to 3-4/10 which mean my ad didn't show at all despite I bid around $1-1.4 per click.