I bought some when DS was a couple of months old and have tried them a couple of times with no success.
For a start the non feeding side fills with milk each time, and if you're lucky this is only a problem when you need to take the lilypadz off to feed on that side and have to catch the milk in something. If unlucky it explodes out itself, soaking you...
If they do control the letdown for you how do you manage feeding in public? Where do you put it while feeding and how do you get it back on again afterwards?
They worked for me, in that I didn't leak milk into them (can't remember exactly how old DS was when I started using them but my milk production had calmed down!) However I am largish of breast and they didn't really fit my shape so would give me weird pointy bulging bits on my boobs so I would only use them while in the house with no one else around!
It was quite gutting actually as they would have been fantastic for me otherwise (although possibly rather fiddly for feeding in public).