Hmm based on my sister's experiences its good for some people but not for everyone.
(I should explain that my dear sister went there 10 years ago so this may be slightly out of date but the general principles are the same. We as a family moved to Wiltshire a few years before and so werent locals as such: we both attended local primary schools)
Academically and facilities wise its very good - my sister thrived and achieved top A levels so I guess it did its job. However she wasnt majorly sporty so didnt benefit from that side of things and at times resented the emphasis placed on sport. She was the uncool studious type rather than the gregarious sporty type.
She felt very contricted by the school as a day pupil.
First its the length of the days
We lived in a nearby town and so she got a coach there every morning - 7:30ish. She would come back home at 6pm every evening having done prep etc at school.
Then its Saturdays
As you are probably aware there is school on Saturday mornings and so on Saturdays she caught the coach as normal 7:30 and then was picked up at lunchtime if she wasnt doing sports in the afternoon. This really affected her as she felt that she never had proper weekends as effectively Saturday was out. It also affected us as a family as we couldnt go away for the weekend in term time.
And its the general location
The school is effectively in the middle of nowhere so there is no scope for pupoils to go off site at the end of the day/ other times - everything is dependent on getting lifts/ some public transport but there is no easy way of just walking anywhere. The closest towns - Devizes and Salisbury - are 15-30+ minutes drive away. As a result of this she felt it was very insular - there was no interaction with other people in general on the way to school etc/ no chance of knowling other people in the local area due to the long hours.
Then its the general people who attend
As others have said its core clientelle especially for day pupils is richer local people/farmers etc - there is a certain culture in the school which she and my parents found a bit limiting (I should explain we werent a wiltshire family and had moved from the South East so maybe we felt it a bit more than some). Of course the boarders are a different type as they come from across the country (and the world) so they are far more diverse.
Personally I took the entrance exam and was offered a place at Dauntsey but chose to go to a day school in Bath which I think was a better choice (certainly for me and comparing to my sister's experiences) as I could travel independently to and from school and also at 4pm everyday I could go home and my Saturdays were my own. The range of families attending the school was also far more diverse and more outward looking - lots of 'professionals' - lawyers, doctors etc as well as university staff and others with a wider worldview than some Wiltshire people.
Thats my experience anyway...