Two of my 3 DDs went to the Steiner school in the village we then lived in - but only for parts of their school careers. The school is now funded by the state - the first in the country. I also taught singing at the school for a few years.
Like most things in life there was good and bad.
The education was established by Rudolf Steiner, based on his philosophy/religion "anthroposophy." It is broadly Christian and theist. I did not buy into any of this, but recognised that some of the educational practices that stemmed from it had great merit.
He believed that children are "incarnating souls" and as such should be nurtured tenderly and not thrown into life too harshly. He would have been horrified by our phone culture and its influence on children. Children at these schools are not taught letters till they are in class one at the age of 7. I can see no harm in that, because what they pursued in nursery and kindergarten was wonderful - my DD danced, sang, painted, went on nature walks, baked bread, learned to knit, enjoyed festivals, made little felt gnomes, listened to stories - she recalls all of this fondly, and indeed makes little gnomes with her children, which they place on my OH's grave. Some of this is being adopted by mainstream schools now in the form of Forest Schools, and the primary where I am governor has a pizza oven built by the children.
DD3 left when she was nearly 8 - she was at Brownies locally and was interested in the local primary school where some of the others went - she asked if she could see it, so we of course said yes - she went along and thought she might like it so she went. She was ready for something new and did well there, but has no regrets over the joyful time she had at the Steiner school.
I have enormous sympathy with the idea that we should not start formal learning too early - it is quite unnecessary. And children do need the chance to be children and spread their imaginations before this gets quashed. At a Steiner school the children have more formal learning at 7, but even then it is linked to art - as is the bulk of the Steiner curriculum, which does not make the same distinction between science and art, but sees both as interconnected. The science books in a Steiner school are beautiful. There is a real sense of wonder, rather than just dull facts, and there could be poems in science books.
The children do take on basic national curriculum subjects: languages, sciences etc., but it is presented in a more imaginative way - always with fluent speakers and often nationals for the foreign languages. And the local Steiner school has an extensive computer suite where the older children learn all the stuff that is taught in a state school. There is an emphasis on beauty: the computer suite is built of carved wood with the computers behind the gently curving wooden doors - the idea of avoiding what is harsh and crude is central.
Music is also central - they all learn recorders from tiny; and they all take part in "Eurythmy" which is basically dancing to music.
DD2 went there at the age of 9 because mainstream school was destroying her. She was a bit of a late starter academically - a degree of dyslexia - and was thoroughly miserable, as were we all with this sad little soul at home. The Steiner teacher said he was not planning to try and teach her anything formal to start with but to let her relax and get over her bad experiences, and play to her strengths: she sang, painted, did drama etc. and began to blossom, which was when them academic stuff kicked in. At 15 she went to a rural comprehensive where she took her GCSEs (she was not behind in any way there), then A-levels at sixth form, a degree at a Russell Group uni and an MA.
All Steiner schools in the UK now take GCSEs and go on to colleges - of DD2's cohort there are plastic surgeons, a brain surgeon, arts outreach workers, theatre directors, scientists, entrepreneurs etc. In no way were they held back academically - but they were streets ahead in confidence.
Steiner schools are as one might expect hot on environmental issues.
There are some downsides of course (as there are with state schools). I can only speak of the school where my girls went of course. There is a bit of a tendency for the school community to be cliquey - to feel that what they do is the only right way - and there is a bit of a hippy vibe. The education does not suit all of course; but it is not the airy fairy do-what-you-like place that people might think. The curriculum is very structured (not to say rigid in some ways) and if you went in to any Steiner school internationally the same year group would be doing the same things in the same term. There is a tendency for the school community to stick together, and I think some parents kept their children there in order not to lose the community and what it meant to them - our view was that we had to look at our children and what was best for them at the time. When my DD2 left one parent said to me: "What will you do for a social life now?" - suffice to say, I coped!
A long discourse - but you did ask! On balance my experiences have been good, and I am sure that your GC will gain a great deal from their education.