We did Palermo in 2011 and that was lovely. Olbia I would imagine is less interesting, because DCL originally went there in 2007 and never returned again until now.
I'm sure you are right, but in my specific case, I ended up liking Olbia more than Palermo. We found Palermo so boring that once we got back onto the ship we immediately booked a ship excursion for Olbia so that we would have something fun to do instead of walking around a dull town again. Worked out great for us, but the cost was high.