We have just purchased 50% of a property and pay rent on the other 50%. The same as Tori once a year is up we can buy more shares and staircase as they call it. If you want you can buy 10% at a time depending on how much money you have. Things to look out for are new builds, they are always more expensive and usually have a monthly service charge. Also after a few years they are not classed as new builds anymore and can decrease in value.
Our house was built in 1915 and it's brilliant! A 50 % share cost us £60,000 and we pay £105 a month in rent. For us it's cheaper than a private rent! If you want to buy more shares you can just add them on to your mortgage or if you have cash I can imagine you could pay outright for the shares. Any alterations you make on the property that adds value is yours say you build an extension, you just ask the valuer to discount the extenstion and they value it without that and then with and you make the profit.
Selling seems to be a lot easier these days, a lot more people are wanting a shared ownership property. I would look out for good areas etc so you have more chance of selling. Our house is right opposite a huge park and playing fields so it appeals to a lot of people! We're also close to town but not in town. When we applied for this place there were about 6 other people wanting to view it and we phoned on the day it was put up for sale!
It's been brilliant for us and has got us on the property ladder even if it is only with half a share but without it we would have been private renting. It's definately something to look into!