Pil,
Good topic.
I've been in two groups and the thing I will say is decide what you want before you join a group. I was lured by £40/ Month and £40/hour. When I worked it out a hundred hours over 2 years actually cost £70/hour. There's no point in buying in and then flying 5 hours per year, or 10 or maybe even 20. Some people in the groups I've been in have never flown a single hour in it. On the other hand if you intend flying 100-200 hours per year a group (or even your own ac) could save money.
I now tend to rent aircraft and do about 40 hours a year, its a bit more per hour but when you squawk the Horizon/DI/radio stack etc you know someone else is paying that bill (and its never less than £1,000 a go is it?). The availablity is usually good and if one ac breaks before your detail you can generally then get into the next one in the line and do your detail. When you own an ac and it goes tech you don't fancy renting one.
This of course depends on there being a good school with a good range of AC to rent nearby.
Aircraft I would go into a group on again are the ones you can't easily hire , the Yaks, Pitts, Tiger moths etc. But if I could hire them I would.
Someone once said,'If it Flies, floats or Fu*** its cheaper to rent it'.After 10 years of flying I am beginning to realise the wisdom of these words.