its impossible to give a 100% conclusive answer to this question, but I can give the evidence that has leads me to believe they work
I have leafleted for 10-12rs, constantly throughout the year, Until 2 years ago I thought if the response from the leaflets at least paid for themselves in Jan & Feb I would be happy, then 2 years ago I designed the
'three special offer' which I now use. the response was fantastic, so good I decided that the three special offers would be the future for my marketing ( I now use them in 2 of the YPs)
So..... before the special offers Jan & Feb were rubbish, but using the special offers (and the leaflet was going to the exact same area and delivered by the exact same person) I was getting a fantastic response and the only difference was the leaflet had 'special offers'
I'll continue this post later.....going to the sales

Mike