We had one a couple of years ago, a 50% share in a house and paid rent on the reamining 50%, we also had to pay a standing charge to the housing association who cut the grass to communal areas.
Selling our share was really easy, we had to have a independent valuation and then we had to offer it back to the housing association for 30 days to see if they could sell the share, they basically posted out leaflets with a photo of our house to anyone on their list and we had about 6 viewings, each person that was interested had to submit forms to the housing association with proof of affordability and the housing association decided who could have it. We were moved on within 3 months.
I, personally, think they are a really good idea and with the prospect that house prices will start to rise again now would seem a good time to buy your share plus with interest rates been low. At the time we had a 100% mortgage although I believe that you wouldnt be able to get this now and would need a deposit. HSBC definately were lending on shared ownership at that time, I believe Halifax and Nationwide do too.
Good luck x