I posted on this thread ages ago and would love to update.
My DS was waking every 45 minutes day and night from birth until 6+ months old. It was hell. I was an exhausted, hormonal mess and suffered with PND. DS was no better and was screaming during his waking hours. Every time he woke up, he would only go back to sleep by being breastfeed (with me sitting up). My husband couldn't get him to sleep with a bottle. I had put it down to colic and did everything I could to help him. I cuddled him, co-slept, and showed him so much affection, but none of it worked.
At the end of my tether, I tried Ferber. The first night, my husband literally held me down while I waited for the timer on my phone to beep (three minutes , then five, then seven, then ten) so that I could go reassure DS that I was coming back and that he was safe to sleep. Within a week, DS slept through from 7:30pm until 6am.
The thing is, it wasn't just DS's sleep that improved. During the day, his 'colic' lessened significantly. He started smiling and giggling all the time, and he started hitting milestones much faster. He suddenly had the energy to try crawling, and wound up speaking very early at 10 months. I also started truly enjoying being a mother and my PND took a turn for the better. There is a reason sleep deprivation is used as a form of torture.
DS is now a clever, happy, energetic 4.5-year-old and has a very secure attachment to both me and his father. He still sleeps through and the only times he wakes during the night is when he is ill, at which point I co-sleep with him.
The Ferber Method saved my family and I am so glad I ignored the posters on Mumsnet who told me that I was abusing my baby for teaching him to sleep.
I would also like to say that I never used Ferber with DS2 because he only ever woke from teething or illness, and it was unnecessary. Some children just "get" how to sleep and don't need help.
Good luck to all of you mums with babies who struggle!