ISF-Day I find to be good with most DirecTV programming, especially sports (Mets games). I only use "Day" on BD with animation (sometimes have even used "Performance") and occasionally with nature documentaries. But there are as many people that swear by Pure, so I think it's all a matter of preference.
Once my TV hits the 1000+ hours mark, I may find myself using "Day" in my dark living room a lot less. Currently, Day can give certain source material a nice "pop". When I find it "too bright" I've been using "Auto". Sometimes "Night" just looks a little too dim for me.