"Do you actually save much fuel in real world driving?" - yes, they harvest all the energy they can from braking etc and put it into the batteries. Of course itr depends how you drive it but yes it should do.
"Are they more expensive to service?" - yes a bit more.
"Do they hold their value, or does battery life cause resale problems?" - yes resale is a problem, potential buyers will be very weary of having to replace the batteries which can be thousands.
"Are you risking problems buying one that is a few years old? " - same answer as above.