Echo what is said here.
Also, if the market looks like its about to tank - avoid like the plague. (big hint here)
Shared ownership really only works in a market where prices are increasing. If they are declining you risk the negativity equitity trap with steriods. Particularly if service or rental charges go up year on year.
I've heard of people getting stuck because they are only allowed to sell at the price the housing association agrees. This means no ability to take offers or drop the price easily. I know of one case of a person trying to sell for 4 years because of this problem.
There's also the sublet issue. If you need to move for any reason, if you can't sell, you also can't lent the property out. We had neighbours with this problem.
Another neighbour had problems but she did at least have the security of being able to stay in one place and not move around with her daughter in rented accomodation.
We bought in a nice area, rather than looking further afield in 2007 at the height of the market, anticipating a crash. It worked for us, but only just - because we paid extra off the mortgage, staircased and because DH's salary went up so much in 10 years to counter balance what the property lost in the crash (we staircased at the bottom of the market and sold at about parity with the original price). We were very much the exception to the rule, and our experience makes me very hestitate about recommending it, particularly in the current climate.
It definitely has its place, but you need to go into it with your eyes open and in all honestly, if there is an alternative or you can buy a house outright, thats what I would do with the benefit of hindsight.
Its probably only really a good option for those who would otherwise be renters forever otherwise in all honesty.