I personally think the flags are a waist of time, money and drag. The flag tends to flap straight back in line with the direction of travel thanks this gets more pronounced the faster you ride. When I see a cyclists and they have a flag I usually don't see the flag until I get much closer. Second problem is that the flags are orange. It has been shown in research that the color orange is not all that visable in lower light conditions (the same conditions that make cyclists less visiable on the road). This is why hgihway crews and other constuction works have switched over from wearing orange only to most now wearing H--Viz lime-green/yellow shirts. If you want to be seen wear bright clothing. Your body is large and putting on something bright will make you MUCH more visable. I ride year round and starting in the late Summer and continuing to Spring I wear an ANSI Class II vest all the time for my morning and evening commute. During the Summer I''' only put on the vest if I am out riding late in the evening or at night (vest also has large reflective bands). When I have seen other riders wearing ANSI level brightness Hi-Viz I can usually see them from a mile out. Usually you first notice a bright blob in the distance. Only when you get closer can you tell it is a cyclists. Compare that to someone wearing dark clothing but with a "safety" flag, and you won't see them till you are much closer. A flag won't hurt, but bright clothing will help a lot. Sure I look like Fred Fall through Spring (a Plant Bike Super Flash adds to the Fredness), but at least I know I can be seens from quite a ways out, even in the dead of night.
Happy riding,
André