I have 1911's with the stock round slide, with a serrated round top, with unserrated flat tops, and with serrated flat tops. My preference is for the serrated flat top. The serrations do seem to reduce glare, especially on hard chromed slides. On blued slides, I have really noticed no difference. I very much like the aesthetic of a serrated flattened slide.
Flattening the slide top also makes the front sight appear taller. I seem to pick up the front sight a bit quicker on pistols with a flattened top.