I'm feeling confused about this at the moment. My first DS is 16 months old so we haven't put him into nursery yet. My DH is a SAHP so there's no need to from a purely childcare point if view.
I had just assumed that we would send him to preschool at 3, when we qualify for the free hours. Is this what most people do? I mentioned it to a colleague the other day, who proceeded to give me a pretty hard time about it, saying "that's the absolute worst thing you could do"!! She reckons its really good for them to go earlier, her daughter went at 18 months and she says she thrives. But they have childcare issues, which is why she went in the first place, so part of me thinks maybe she is so positive about it because they had no option iyswim?
I try to ignore people's opinions for the most part, and do what I think is best, but it's hard sometimes. She seems to have made me doubt myself, she even said "I'm not trying to say you're a bad parent but..." 
I did always say that I'd play it by ear and wait and see how he is, if he seems to be getting bored and I think he would benefit then I'll consider sending him. I'm just not sure now what to do for the best!