Hi flygra
I know what you mean about colds i have found the same and have to go of sick each time i catch one as my sinuses and ears completely block and I have done some damage to my eardrums flying like this. So I take echinacea and i do think it works, if you take it solidly for 2 months then have a couple of weeks break.
It surely cant do you any harm unless you never have a break from taking it, which may cause your body to get used to it and it wont have the same effect.