My dcs went and I loved it, they are 4 and 6 now.
When I looked into it I was told montessori's vary enormously and to judge each one on it's own merits.
We had a fab one, my DC (ds 4.11 and dd 6.6) were well prepared for school, good table manners/general manners (when out without me!), good at tidying up, can put their own clothes and shoes on (montessori focus on life/home skills), confident at maths (another focus), had been introduced to phonics/letters, wrote their names (and DD a bit more). I am glad I sent them.
I paid approx £1500 - a year. I used the child benefit to pretty much pay for it. You can used your nursery grant, that paid for about half the fees.
It was lovely.
Drawbacks:
It was 8 miles away and DD didn't know many people at her Primary school when she went.
I felt a bit odd pulling up in the car park in my 10 year old banger, among all the BMW X5's, astons and audis (but it was in a posh village area). There were about 4 parents with normal cars, but I think mine was the oldest/most battered!