i have had my S2 since june 3rd 2011. I have had the SGP Ultra Silke Soul Black Case on it for the first 2-3 months. however, after getting the 2000mAh extended battery, i removed the case and since then have been using it without any case.
the screen does not have any scratches(neither does the screen guard), however, the silver plastic around the screen has had a small dent near the top right of the phone due to it falling from my hands onto concrete(ouch!)
Since then, i am using the standard samsung flip pouch with the phone!