DS2 has done 11+ years of full boarding, tonight as I type he's got five weeks and 1 day of boarding left (not including 1/2 term).
Firstly to pick up on some comments above:
"I came out at 18 without a close relatinship with my parents, my support network was my friends".
I have an incredibly close relationship with my DS, in fact he's just written to me saying how wonderful it is that we are so close and how he feels he could tell me anything and how he knows I'm there to support him.
"everyone rolling in money and wearing designer clothes"
I'd be the first to admit that many are rolling in money even if like DS2's school you have quite a few on bursaries and I'd also be the first to admit that when it comes to expensive designer label clothes I'm a bit clueless but DS2 assures me that at his school most are not clad in very expensive designer label clothes just the usual sort of stuff that the MC wear and most of the parents are a pretty scruffy bunch I always think, lots of expensive cars though. It doesn't bother me but it might bother some and I'm sure the wearing of designer label clothes varies from school to school.
"There will be ocassions when they might need you and you're not there"
On the couple of occasions when my DS needed me I was there within a couple of hours, no different from home really.
So these are my pros:
Endless opportunities to enable my DS to receive a wonderful broad intellectually stimulating education, there are 30+ concerts a term, at least 8+ plays a term, twice weekly lectures from eminent outside speakers on a huge of range of topics, all free for him to attend if he wishes, numerous sporting and non sporting societies probably 60+, a wonderful opportunity to do his own sport 3-4 times a week on his doorstep unlike at home where is a 70 mile round trip, at his school all the teachers (dons) live around the school so he can access help when he needs too, this also promotes an excellent relationship between the boys and dons, he works, plays, eats, sleeps with 59 other boys from a wide variety of countries some he likes, some he doesn't, some are untidy, some meticulous, some are irritating at times etc to board successfully he has learnt to make the best of it, to accept difference in all its shapes and forms, he has become exceedingly adept at reading situations and adapting accordingly (apparently he's a great house guest), he's learnt to be part of a team and that the world does not evolve around him, to shift for himself and to not be afraid of unfamiliar situation, I hear friends say I can't sleep in a strange bed or even more bizarrely use a strange loo, few boarders think like this. Hes learnt in a very busy life how to find and create privacy for himself, also to know when other are wanting privacy and when they're not, living so closely alongside others he knows it's not designer clothes or expensive cars or large houses that make you a decent individual. I personally think these are very important life skills. My DS has lots of freedom at school more so than at home because at home we have no public transport system and there no where locally to go so he has to rely on me to take him anywhere. Very strong friendships are formed. You don't have to walk far to find friends and activities you might be interested in but this can be a con during holidays because hes used to everything being on hand. You can't be a helicopter parent. You won't know what's going on all the time and you're having to entrust the care welfare and education of your DC to other, some could see this as a con. You'll love the first day of the holiday and start counting down the days to him coming home about two weeks in advance, the day before they break up you go around grinning like an idiot (could be a con your friends and neighbours might think your mad). There not eating you out of house and home 52 weeks of the year it's someone else has that problem.
Cons: it's bloody expensive! I miss him everyday. He misses us. Travelling back and forth to pick him up on exeats, holidays etc. No space. Little privacy. No friends close by in the holidays. School meals even if they're great it's still mass catering. Relationships between boarders and staff can be brilliant, the line between pupil and don is very blurred and this creates excellent relationships, but you could be unlucky with your house staff in particular, your DC or you might not get on with them or agree with them but these are key people in a boarders life. Boarding is exhausting, DS2 is absolutely exhausted now. When one gets a cold or D and V or whatever many others will come down with it. Your DC however organised will always loose things, socks being a big one. When they come home for holidays you'll suddenly find that you have 1/2 a ton of washing to do! You'll hate the last day of the holidays. You'll need to be flexible enough in your job/life to pick them up drop them off at awkward times.
I'm sure there loads more pros/cons but this us all I can currently think of. My DS and I have talked about it recently neither of us regret our decision to send him to a boarding school.
Good luck OP.