I've taken Jadrolinjja but only as a walk-on day passenger (Dubrovnik-Bari FWIW). Other than facilities offered on various ships (generally all the same though some may have sun decks, swimming pools and night clubs - Jadrolinija had recliner/sleeping seat lounges ($)), they're pretty much all equal in my experience (drive-on large ferry crossings include 3 x Corsica Ferries, + 1x Tirrenia), 3 of which were overnight (we had a cabin for each - which are pretty small and basic).

If you haven't taken a ferry, it's checkin at the terminal and wait in the lot. Ferry may already be there (generally large stern-loaders). Small vehicles may be stowed in the under main-deck hold. One cannot remain in/access the enclosed vehicle deck(s) during sailing so make sure you bring all your stuff with you that you need on the voyage. You won;t be able to return to the vehicle until the ship has pretty much docked. Be prepared for delays. One of the last crossing we did (Savona-Bastia), the front of the ship wouldn't open on arrival to unload so they had to turn the ship around and everyone had to reverse out or do a U/3 point turn. Next large ferry (Tirrenia), the ship was late arriving at the intermediate point of Arbatax (stop between Cagliari and Civitvecchia).