I did Mountbatten a year and a half ago and had the time of my life. I can't recommend it enough - it was such an incredible year. Fantastic doesn't even begin to sum it up.
I was worried about sharing a room - don't - it's fine. You just get on with it and the rooms are mostly quite spacious in New Jersey so privacy isn't too much of an issue. Rooms in Harlem vary depending on your apartment but it's doable - frankly, you don't spend a lot of time sleeping anyway. They do match you very well with roommates - hardly anyone had huge problems and if you're really not happy you can move at 6 months.
The PGC qualification is a joke - I did no work and got a Distinction. It's business for dummies and no where near an MA qualification - no one takes it seriously and no one actually studies for it. You have to go to one two hour class one evening a week and write one essay per module - and you can miss three classes per module without being penalised so it's hardly taxing.
The money you get to live on is pretty tight but everyone's in the same boat and most New Yorkers are poor too so there are a huge range of free activities and events happening all the time. I took out about £2k worth of savings and that was fine to last me the year. I lived life to the full and did a lot of travelling - it's not as if I was at home every weekend. I will say this though - alcohol is expensive especially as you have to tip $1 for every drink you buy. You will never have any spare cash if you go out drinking every night so stick to drinking on the weekends and you'll be fine. Eating out is fairly cheap but buying groceries isn't - you'll soon find that it's cheaper to eat out than cook especially if you like veg and fruit - they're especially expensive.
If you want an adventure and you want the opportunity to live in New York then GO - don't worry about all the sharing rooms, money, PGC stuff - it all becomes totally irrelevant when you're there and you get to walk the streets of New York every day. It's the best decision I ever made, the best year I ever had, and I often wake up after dreaming about New York and am practically crying at my longing to be back! Do it do it do it - and YES there are finance options available - many people got career loans. Ring up the office in London and they will talk you through your options.