I did. Twice! Both dd1 and ds started into primary 1 at age 5y6m, and it has been a rip-roaring sucess for both of them. Personally I just don't understand the need to get them into school so young - they're going to be there for 13 years, why start so young? My 2 would have coped, but the extra year in nursery was good for them, especially ds, and has given them extra confidence and social skills to cope etc.
There were a lot of considerations - they will now be 18 rather than 17 on leaving school, they will go to secondary at age 12 rather than 11 (and as a secondary teacher, again I think that is a great thing, the kids who struggle with the transition are often the 11 year old boys), they had an extra year in nursery which was far more play based, I had an extra year with them, which was lovely, and I think they have settled in well. THey'r ealso in bilingual education (english/gaelic) and the extra year of gaelic nursery built their vocab more before they had to go into a more structured setting.
I would always say defer. I can't do it with dd2 as she has a nov birthday and so will be 4y9m, but then I think she will be ready, and I think it is more of an issue with boys.
In Scotland you register for primary school in January, so you have a bit longer than you think to think it over. You also can register and then withdraw if you ds isn't ready.
I know quite a lot of people who did this - its kind of the norm where we live, and I dont know anyone who regreted it, but I do know some people who wished they had deferred. The last term before school starts with an energetic 5yo can be pretty boisterous though!