There have been a few threads in the past on "sharing", including RIB Shack. Some people hate the idea. One thing to remember is that most of the people on here like the idea of owning their own boat and using it when it suits them etc. That means that many of them have selected themselves out of the RIB Shack target market. The small number of people who have used RIB Shack and posted here, have all to my memory, had positive things to say.
Depending on your usage levels other options might include bareboat charter (e.g. Solent Rib Charter at Plymouth) which could give you something like 10 days afloat for the typical cost of storing, insuring an servicing a large rib.
Or how about these folks, not exclusively ribs, http://www.themarineclub.com/
there are a couple of other RIBshare schemes which are more similar to RIBshack to consider too e.g. Lynx www.lynxsports.co.uk
there are others too...
One thing to be wary of is they all seem to try and make out they are "cheap" - which I think is misleading. Less expensive, maybe, cheap no. e.g. I saw one advertising something like £20 per person per day. Yes if you got 10 people in the boat and used it every day you were allocated then that is what it worked out at - but the reality is you won't use every day you are allocated, and you probably won't fill the boat when you do. And very few people will expect their "guests" to contribute - so the split cost model is marketing irrelevance.
Having had the pleasure of being a guest on a charted Rib earlier in the year (thanks again John
) and not having as much time afloat as I would like because of family/work committments etc - I can totally see the benefits of an occassional use model, with preplanned time on the boat. Whether you still need a small boat to potter round in on the rare beautiful days when the boat is not booked to you is a matter for debate.