I'm thinking of putting DC into Milbourne Lodge as well. I've done some research on the mumsnet board, plus read the entry in the good school guide, ordered the school prospectus, even read Chris Woodhead's book(chairman of Cognita), in which he lavishes praise on the school, and lays out his philosophy of not changing it's ethos.
However, the school lacks in top notch facilities (compared to Parkside, Cranmore, Daneshill etc). Also it is much smaller in terms of student numbers, 180 compared to around 400 in others.
But what matters to me is the fact that they've managed to put ALL of their 30 odd class of 13 year olds in top notch schools like Eton, St. Paul's, Harrow, Tonbridge.
Regular winners of King's Scholarship at Eton infact.
And they have been doing it successfully for the past 50 odd years, so definitely not a fluke.
Some parents i spoke to were worried that the standards might slip up after Cognita decided to buy the school in 2007, but that hasn't been the case (so far).
Head is new though (only there since September 2010), but has strong views on the school's future (no let up on academic standards).
So in summary: an excellent (and some say academically very pushy school), but it all boils down to personal preference.
They have an open day in May. Go have a look if you like it, despite the lack of glitzy infrastructure.
Hope this helps.