Current parents here.
Who goes: The mix was getting a bit too international a couple of years ago but it's heading back the other way following an influx of local day boys into the junior years (has lured a lot of families from St Hugh's, who see it as a definite upgrade). Remains very popular with British army officers who love the fact that there are so many full boarders, and with British families who want their sons to get used to boarding before going on to hard-core full boarding senior schools like Eton & Radley. Weekly boarding is not an option - you are either day or a full boarder - but with long and short exeats and half term, nobody ever seems to be in school for more than two weeks at a stretch.
New head: views vary a little depending on who you ask. He is a nice man with good intentions - and his wife, who is also very involved in the school, is universally liked. Much more "by the book" than his predecessor, and a bit less obsessed by sport. Has perhaps been naive in trying to introduce too many changes too quickly without thinking through all the consequences or taking parents and boys with him, but he is (a little belatedly) trying to put that right with a new parental consultation group.
Co-ed: It's clearly not going to be the same Cothill in two years' time that it was when we signed up six-ish years ago. We loved the unflashy, adventurous, let-boys-be-boys feel of the place, and rather sadly that will go once girls arrive. Girls are much more sophisticated in Y7/8, and it's a shame that in coed preps boys end up under pressure to grow up before they are ready. However, the die is cast and girls it is. Hopefully some of the distinctive Cothill magic will still remain!
Senior schools: DS got into the (over-subscribed) senior school he wanted at first time of asking and we have no doubt that having attended Cothill made a big difference. Public schools like boys who know how to board, and there's no better preparation than Cothill. Having said that, while a very high proportion of Cothillians go on to Eton/Winchester/Radley/Harrow, it's not an automatic ticket. If they think a school is unrealistic they will tell you, and then move heaven and earth to get your son into the best possible alternative.
@Nottodaythankyouverymuch LMK if any other qs.