I've used it for over 3 years now and have noticed I don't get sick when everyone around me does. During the flu season, I take it every day and also do that when I travel. Right now, I use it every other day, but will take it every day at WDW.
I prefer the lemon-lime flavor and take only one a day unless I feel "sniffly". I drop one into a bottle of water and add a Crystal Light flavor packet - orange, lemon or cherry pomegranate. I drink it leisurely. There are other flavors - orange, pink grapefruit and I just saw hot apple cider flavor at BJ's. I also get a flu shot once a year, but ever since I got sick w/ a nasty cold while at WDW, I've been trying things to boost my immune system. This seems to work well for me.