Our red bin is pretty much always full now, but that's because our garden is infested with cabbage trees so now we can just stuff all the bloody cabbage trees leaves in the red bin. Yellow bin seems to get pretty full (which makes me wonder why we need so much packaging in our house and has made me look at ways of reducing how much rubbish we actually bring into the house, so a good thing). We have a compost bin so green bin is pretty empty most of the time, unless we've been gardening, but it does get quite stinky since meat scraps do tend to go in there.
I was also sceptical of the bins, but think they're bloody good, but we have very convenient places to store them so that makes a difference.