Echineacea is good for?

I bought a BIG box of Echineace today, I'm determined to build up my immune system!
( I have asthma and religiously get sick every 2 months with chest infections)
Will Echinacea do what it says on the box?
Does it work?
How long do I have to be on it, to know its kicking in?

Thanks :-)
Update: Well, thats me totally confused then! Some say its great some say it's not. There is really only one way to find out I suppose. Thanks to everyone who took the time to advise :-)
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