We sleep trained our son at 7 months. He cried for 90 minutes the first night, then slept soundly until 7 am. The second night he cried for 45 minutes then slept until 7 am. He didn't cry the third night. Every night since he's slept peacefully from 8 PM until 7 AM and asks to go to bed when he gets tired. He's happy during the day unless something obvious is distressing him. Most notably he's happy and peaceful from 5 PM until 8 PM when I see other parents suffering from witching hour and unexplained melt downs.
Isn't a few hours of discomfort in exchange for years of good sleep and happiness a good trade off?