They are the greatest invention since sliced bread! Not that I'm biased of course!!
They will last just as long as a hard boat, they are just as durable as a hard boat (although your toobs will need replacing once every 10-15years if you look after them....but not a great expense in the long run), they can't sink like a hard boat does, they make better and more stable platforms at rest and can handle some of the roughest conditions you can throw at it, and they are more fun.
They are generally quicker than most hard boats, have good all round visabilty, they are easier to sail and they come with a large built in fender!
The only thing they don't offer without great expense is somewhere to sit out of the rain and wind!
As for holding their value? Over here they tend to hold their value quite well but then there is a relatively big market for second hand RIBs and it may well be different over there.
Personally, I will become a hard boat owner eventually but hopefully a big enough hard boat for a RIB tender!!