Yes there are a few of us on here who own shares.
Advantages are that it's significantly cheaper than hiring, you get to know your own boat, and you have a real sense of ownership.
Disadvantages include not being able to go out on the boat at the drop of a hat if a sunny weekend comes along.
There are several schemes, all of which seem to do things differently. Things to check: Do you (and the other owners) actually own the boat, or is it just time share; is the allocation of holiday weeks fair; can you get extra weeks, and how are they organised; how easy is it to move mooring location.
I know there'll be people along shortly who say they'd never go for shared ownership. One day, we too will have our own boat, but while holiday time is limited it works for us as a cost effective way of getting on the canals.
If you have specific questions, feel free to PM me.
Adam