To answer your question, yes a good idea. One of the best decisions I made in flying was to form a group.
There's only 3 of us, we all get on, we fly with each other more often than not and we have similar flying aspirations and ambitions. Now we may have got lucky here but we've been in this situation since 1998 and apart from a few unexpectedly expensive bills it's been plain sailing. I guess we all knew what, and who, we wanted to deal with so it was worth the patience and planning to get it right.
The best bit is the availability aspects, popping out for an hour of an evening or a weekend afternoon, just arrange it amongst ourselves, no over zealous booking procedure to worry about - and we agree to keep her for longer trips (we made the Med a couple of years ago!).
We've invested in the aircraft along the way in terms of avionics and although she's an Aerobat (Reims FRA150M) we mollycoddle the engine and do that sort of flying in someone else's machine! So touch wood, we hope to keep her going for a long while yet.
Having said all that I have seen other groups have troubles but we've steered clear of these. Sure, we could do with more money in the kitty but we run it not to make a profit (something I think we're particularly good at!) but we aim to keep the flying as cheap as we realistically can.
Overall I think it's been very beneficial to my flying, I think of it largely as "my aeroplane" (well a third of it) and it keeps the enthusiasm up, time permitting. I don't think I could have done what I've done when I did it just by renting a club aircraft, something I need to bear in mind if I dare start to take it for granted.