There was an advert on the tube claiming echinacea could reduce my chance of developing a cold by 65%. Blimey, a big claim. So I went and found the source of the cliam, and a couple of other review papers. My summary of the research is this:
There are all sorts of caveats on this summary. Firstly it's not recommended for children, or for people with immune problems such as arthritis or HIV, or people who might have an allergic reaction. Secondly we basically don't know how it might work (it contains a few chemicals that probably interact with the immune system... but in what way?). Thirdly we need more big studies before we can be sure about the effect on outcomes - so the picture might change, might even change dramatically, as more science gets done.
But I want to emphasise: in terms of real-life evidence, echinacea has much better evidence than homeopathy, or than other herbs or other such stuff you might find in that same section at the chemist's.
I noticed that some science bloggers tried a little bit to poo-poo the meta-analysis, and to be blunt I suspect that's because it finds quite decisively in favour of echinacea. (None of my favourite science bloggers had this prejudice: David Colquhoun and Ben Goldacre are my favourites by the way.)
I do personally have an instinctive scepticism of complementary medicines because of the way things often try to side-step proper evaluation while at the same time giving themselves a white-coated pseudo-medical image. But in this case I'm happy to say that both the reviews find generally that there is a positive effect on cold from (some) echinacea preparations.