The value of a share is ultimately only what someone else is willing to pay for it (and that principle goes for the whole aeroplane as well of course)
Factors are
Location
Group (size and philosophy)
Type
Fringe Benefits
Aircraft Value
Whatever you do make sure that the group is the right one for you; if it isn't you may have to write off your investment.
I (as have most people) have only positive experiences but judging from what one sometimes hears it can be an awful experience if there is a massive fallout (including going to court etc)
Yuk