A share is a very good way to cut flying costs and/or get into a plane of reasonable quality, but again there is no free lunch.
1. You are liable (to the extent of your % shareholding) for all maintenance costs, and big-cost suprise items are normal on old planes
2. There are botchas to watch for. People sell their shares for a reason and they aren't likely to tell you if the reason is for example advance knowledge of a major expense, or a dispute within the group. You have to really know and trust the rest of the group.