We both felt sick to our stomachs the first night we did it. DD has had brilliant headcontrol from the second she was born, and had reflux, which meant that everytime we laid her on her back, the acid came up from her tummy. I read as much as I could before we did it, and decided that, because of other influencing factors associated with SIDS, putting her to bed on her back, while a massive factor was a risk that we personally needed to take.
We hadn't actually had anything longer than an hours sleep from her being born at this stage and we we were more than six weeks in. I admitted it to the HV, who was very worried about me, and she agreed it was worth trying.
My advice would be to look at the influence of each SIDS risk factor for your circumstances and decide if you think it is worth trying. They are guidelines, not laws, and as with all public health advice, what is applicable to the vast majority, and therefore, communicated as such, is always dependant on individual circumstances too, and must be considered as such.